On damage rotations and disease stacking
Despite the extensive changes coming in the next patch, Death Knights do still have a solid way to pay in PvE: Apply debuffs and diseases via Icy Touch, Plague Strike, a Ghoul's Infect, or any other talented method you may have, use Blood Strike or Obliterate to do damage once the diseases are properly stacked, use your runic power dump (usually Death Coil, at least at lower levels), rinse, repeat.
This isn't to say, of course, that there aren't multiple other ways to get your fight on, and depending on the fight and your spec, you may be adding some Corpse Explosion, Pestilence, and Death and Decay in there if you're AEing, or perhaps a Howling Blast or Frost Strike if you're Frost, or any number of other abilities depending on your talents and the situation at hand. However, once you get the rythm of the rune cooldowns down, playing a Death Knight is a lot simpler than it first appears.
As Ghostcrawler has said, they don't want the Death Knight to be so complicated it's confusing, and many of the basic changes next patch reflect that. You can see those explained a bit by our class designer and impromptu Death Knight Beta CM in this thread.
Firstly, diseases are changing quite a bit in that many of them are going away completely. Essentially, there will be two baseline diseases for all Death Knights: Frost Fever, which is the attack and casting speed debuff currently caused by Icy Touch, and Blood Plague, which is the disease DoT currently caused by Plague Strike. Most other abilities and talents that currently cause diseases will now instead cause either Frost Fever or Blood Plague. If you want extra diseases, you'll essentially need to spec into Unholy to grab Ebon Plague and Unholy Blight.
This should in theory make diseases a bit more managable, although there's a few concerns that may need adressing: First, that Infest isn't listed by Ghostcrawler. Considering Epidemic also no longer increases the duration of Ghoul diseases, it seems like it's gone. That means Ghouls have lost a nice bit of functionality in allowing for better disease stacking. Secondly, Blood Strike and Obliterate may be getting nerfed by default, since there won't be as many diseases for them to apply bonus damage to. Both of these are something that may need some testing to make sure they aren't underpowered.
As far as the damage rotation put forth by Ghostcrawler itself, there's some definite changes that should, in theory, streamline how Death Knights DPS. Reducing Plague Strike from 1 blood 1 unholy to 1 unholy rune is definitely a good move, since it allows for 2 Blood Strikes in a row for some nice extra damage -- probably needed, since the amount of diseases has been pushed so low. It also means you can get up your two basic diseases quickly with only one rune each, whereas before, you needed either a deep Unholy build or 2 runes and an active ghoul to get up 2 quick diseases.
What should also help is some of the new baseline abilities. Raise Dead is being made free to cast. Having to use those 2 Unholy Runes was a major pain, since it meant 10 seconds of almost mandatory downtime if Summoning a Ghoul before battle, and ade it almost impossible to use during battle. If there's one thing about this that may prove a little overpowered, it may be chain-summoning ghouls to explode during AE Battles, but the fact that you'll need to either have spare humanoid corpses or a lot of uber-expensive Corpse Dust may balance it out. Death Strike now does 50% of weapon damage as healing to the Death Knight per active disease, so that should make the skill much more useful and make Vendetta much less of a required talent for grinding and leveling.
On Talent Tree changes
The talent tree changes are pretty extensive, as mentioned. A lot of them are simply changing "diseases" to specifically apply to Blood Plague or Frost Fever for various talents. In addition, a lot of talented abilities will now inflict Frost Fever or Blood Plague instead of a seperate disease. There's also been at least a small effort to allow Death Knights to get more tanking ability from talents, with many tanking abilities moved to the lower tiers of their trees and a few new ones (Anticipation in the Unholy Tree) implemented. You can see a full list of the changes on MMO-Champion.
For the most part, Blood has remained the most solidly defined and polished tree in the Beta. It has found a perfet niche as a focused single target DPS tree with tons of self-sustainability and a surprising amount of decent tanking skills. Forceful Deflection, which gives +25% of strength as parry rating, has been removed and made baseline, but there's plenty of other skills to compensate, including Blade Barrier, which has moved from Tier 6 to Tier 1.
A Death Knight tank may want to strongly consider taking a bit of Blood for skills like Blade Barrier, Bladed Armor (now Tier 2, but still a nice way to make AP and armor interact and scale), Rune Tap, and Improved Rune Tap (which now reduces Rune Tap's cooldown in addition to providing more health, and should be easier to use because of the Blood Rune freed up by the new, cheaper Plague Strike). Even a deep Blood Death Knight should be a good tank, in part because of the new talent, Vampiric Blood, which increases all healing recieved by 60% when all Blood runes are on cooldown, even heals from outside sources.
Death Rune Mastery's new function looks like it could be useful: Whenever you use a Death Strike, the Unholy and Frost rune will turn into a death rune next time they activate. Of course, first thing one notices is that apparently Death Strike will require a frost rune. That's probably not too bad for a Blood Knight, since Icy Touch is generally the only frost rune you'll use in a basic DPS rotation in the current build. Unfortunately, it's also both Frost and Unholy Death Knights that really suffer from rune starvation, so it would have been nice to see something like this in the other trees. A guaranteed death rune or two would do wonders for Frost, especially.
Death Strike is also the new trigger ability for Blood Worms, which is the newly renamed but still very similar Infested Corpse. This Blood Worms change might also explain in part why they added a frost rune to Death Strike: It will prevent spamming the ability for instant heals or to proc more Blood Worms, at least if you want to keep up Frost Fever.
Heart Strike has also changed drastically. Gone is the max HP debuff. Instead, it will now act like an upgraded Blood Strike and do damage per a disease on the target, as well as preventing any benefit from haste effects for 10 seconds. It's not surprising to see the max HP change go. Ghostcrawler explained in a forum post that the max hp reduction effect seemed too confusing, with too many questions about when to keep it up and how long to keep it up, and so on and so forth. The new haste prevention effect, however, seems to set it square in the realm of PvP, despite the fact that PvP is generally the purvey of Frost (for the crowd control) or Unholy (for the debuffs). In this case, Heart Strike may need something that makes it slightly more desirable for PvE DPS, unless more mobs and bosses have haste effects that I realize.
Overall, however, Blood is still firmly established as the "vampiric DPS" tree, and the most solid tree at this point in the Beta. It has an established role it can play, it provides quick and easy grinding, and it has the best early tree to cherry pick from for other builds, both in DPS and tanking abilities. The other two trees are getting there, but they aren't quite on Blood's level.
There's some disagreement on how you might describe the Unholy tree -- is it a tree of choices, or a tree of bloat? Especially with the proposed changes, Unholy is a tree with a lot of little talents at various tiers that may or may not actually be worth taking, such as Epidemic (especially since it will no longer include Ghoul diseases). But all those little choices do add up after a while, and it really seems like a few of them might need a bit of consolidation.
One nice thing about the new changes is that it does seem like Unholy may get back some of the AE damage and AE tanking potential it lost when Death and Decay was nerfed in damage a few builds back. Morbidity (which is mislabeled on the current talent tree calculator) will reduce the cooldown on Death and Decay and the runic power cost of Death Coil. Being able to semi-spam Death and Decay again, assuming the cooldown reduction is significant, maybe make Unholy the tree of choice for AE Tanking too. You can also combine this with the talent Outbreak, which will increase the damage done by Pestilence and Blood Boil, giving the Unholy Death Knight even more damage and threat.
Paladins shouldn't worry too much though, the Death Knight still lags behind enough in innate mitigation and avoidance that a Paladin will be a superior AE tank. Bone Armor (which was understandably moved to later in the tree, and will keep the same functionality despite what the current talent calculator says) will help with that somewhat, but at only 4 hits worth of mitigation, it won't really be able to pick up too much slack. Bone Armor is still a talent pretty much any deep Unholy Death Knight should consider taking though, if only for the increased 2% damage while DPSing.
One of the more distressing changes in Unholy is the addition of the baseline skill Degeneration as the deep Unholy 41-point talent Scourge Strike. This makes PvP Death Knights far too dependent on the Unholy tree, when they really do need deep Frost to have any chance of reliably PvPing. Chilblains, for example, is almost required for PvP to have a chance at keeping anyone in melee range.
Of course, the other problem is that Scourge Strike really isn't as powerful as it should be. Rather than completely preventing HoTs from ticking on an opponent as the tooltip would seem to suggest, it instead only converts a single HoT into a new disease -- meaning that the healer can simply reapply their HoTs to the target and be done with it. In the end, it really needs to be made Baseline again and given the ability to block all HoTs, making it the Druid counter it should be.
Finally, a few more Unholy talents desperately in need of a review didn't get one. Anti-Magic Shell, for example, is supposed to be a reactive ability (a 3 second duration leaves it no time to be anything else), and yet takes an unholy rune that is likely to be on constant cooldown in many damage or threat-gaining rotations. Because of this, the associated talents in the Unholy tree are really not worth taking as much as they should be.
The Ghoul created by Shadow of Death has no real survivability or power (almost any monster that kills you will kil your Ghoul in a few hits anyway, sometimes before you have a chance to so much as explode yourself), and is bugged so that you die at the end of a losing duel and become a Ghoul if you have the talent. However, you still have to take simply to get Master of Ghouls, which gives your pet a pet bar. It's an imperfect pet bar that does not include passive/aggressive/defense or stay/follow buttons, and never remembers how you arranged the hotkeys from Ghoul to Ghoul, at that.
This pet bar really needs to be standardized and made baseline, while Master of Ghouls deserves a stronger effect. Since I've spent so many points to make my Ghoul a viable partner and part of my DPS rotation, I'd like to see Master of Ghouls help with that. Perhaps it can remove the need for Corpse Dust, or at the least allow Ghouls to be resurrected from undead and demon corpses as well as humanoid corpses. It seems wrong that such a unique and strong part of my DPS can cost me 50 silver every 4 minutes. Right now, I'm afraid the high cost of Corpse Dust (which should be cheaper regardless) combined with the removal of Infect and the low duration of the Ghoul even with talents will make the poor Ghoul little more than a gimmick in the long run.
Frost still remains the redheaded stepchild of the Death Knight trees in many ways. Since it requires a lot of freezing, it doesn't play nice with DoT diseases, and the freezes and snares are hard to come by and unreliable at best, making crowd control a touchy job, and making any real attempt to snare and catch up with a fleeing enemy or pvp target not as easy it seems it should be.
However, the latest changes look like they should help the make the Frost tree a bit more flexible and managable. For example, Howling Blast currently does triple damage to frozen targets. In the next build it will instead do double damage to any target inflicted with Frost Fever - the new disease caused by Icy Touch. You can simply use Pestilence to spread Frost Fever to multiple mobs, and finish them off with a Howling Blast. You can also still use the old Hungering Cold combination if you wish, since Hungering Cold will also apply Frost Fever. Similarly, Chilblains and Tundra Stalker will now apply their effects to victims of Frost Fever, and a new talent called Endless Winter will cause Mind Freeze and Frost Strike to apply Frost Fever. Essentially, you can snare whole groups of mobs left and right with the right talents and do tons more damage without relying on freezing or specific effects.
Frostbite is gone, but you can still consistently freeze people with Chains of Ice and Hungering Cold, leaving some synergy with Frost Mages still in place.
Frost Strike, which has been shifting back pretty constantly between "on next strike" and "instant" in an attempt to find its place, has recieved another relatively major change in the upcoming build: Now, it's a runic power ability. It should be interesting to see how it fits in to the Death Knight's rotation. At the least, it should be nice for a good shot of armor-ignoring damage, and as a good Frost Fever refresher with the Endless Winter talent.
One change that I am still trying to decipher is the moving of Lichborne from the Unholy tree to the Frost tree. The Unholy tree probably suffers the most from a lack of good tanking talents, so moving one of the good ones to another tree seems a bit off. Of course, Lichborne is low enough that an Unholy Knight can still wade in and get it, and may want to, in order get Annihilation as well. This may be part of a ploy to give Unholy Knights a good reason to go Frost Secondary instead of Blood secondary. Now that Vendetta is not as needful for grinding thanks to the new Death Strike, Frost as a secondary tree for Unholy may deserve a second look.
As a class, Death Knights are shaping up to be very fun and unique, but they could also use some tweaking, as you can see, especially if the PvP and Tanking areas. Hopefully, a little bit more attention can be bought to our PvP plight when Lake Wintergrasp and the new Battleground and Arenas go in for testing (primarily, we need a baseline reliable snare to keep our enemies in range of melee), and hopefully we get a bit more help on the tanking end -- Specifically, I'd like to see our "reactive abilities" such as Bone Armor and Unbreakable Armor switched to runic power so that they can be truly reactive and not dependent on rune cooldowns and more innate avoidance and mitigation overall. Forceful Deflection going baseline is definitely a good start on the latter.
Regardless, it looks like the next build may change the way Death Knights play in some very significant ways, so it may be that we'll have to wait until it's on the Beta servers and we can play it before we know the true state of the Death Knight. For now, I leave you with a few questions and answers:
Is the Death Knight a class in progress? Yes.
Will the Death Knight need to see a lot more changes before it goes live? Yes.
Is the Death Knight class fun? You bet your sweet bippy.