The most compelling choices to be made, for me, are on the level 60 tier, where you get three pretty compelling ways to do some self-healing, each with perks and drawbacks for each spec. Death Pact gives a huge chunk of health based on your max health but requires frost and blood death knights to spend a global cooldown getting out their ghoul. Death Siphon is nice if you have spare death runes but probably won't do near as much healing as your other options in the long run. Conversion could get overpowering just because it can literally last until you run out of runic power, but runic power generation is somewhat nerfed, so that may be a reason it works. It worked well enough for questing when I tried it, and I'd hesitate to call it overpowered until we see how it does in endgame content. If nothing else, it does take away the damage (and most importantly, Blood Shield procs you'd get from the Death Strikes you'd get from triggering your rune regeneration using runic power dumps) you'd get from the runic power you spend, which may in and of itself be enough of a balancing factor.
Next, you'll need to decide what glyphs to use, and I will be honest, they seem to need the most work. Prime glyphs are gone, leaving you with only three major glyphs and three minor glyphs to assign. Luckily, this is pretty easy to do, because none of the glyphs feel like much to get excited over. There are a few that promise some limited utility, such as Glyph of Tranquil Grip, but there are very few glyphs that scream Fun and meaningful boosts to your power level! or Cool and interesting cosmetic changes!
Luckily, this is only beta, so I'm hoping death knights get another glyph pass or three. More cool cosmetic stuff like pet appearance changers or a subtle physical aura on a presence could be cool, as would more compelling glyph choices that feel like they add power or change abilities (in PvE especially), instead of just adding a few more yards of range.
Joining the fight
Upon getting to Pandaria, I found that, for the most part, leveling appear much easier than the start of Cataclysm. The first few quest rewards I received started at item level 372 (that is, just below Firelands gear), but by the middle of the zone, I'd picked up an item level 404 weapon -- enough to eclipse patch 4.3 Raid Finder gear, but I still had only replaced about half my gear.
Death knight self-healing is, of course, still not where it was in the early Wrath heyday, but I think that with the judicious use of your level 60 healing cooldown, you'll find yourself more than survivable against single targets and probably even good to go for a couple AoE sessions, (though those get a little stickier if you're not blood or didn't take some of the heavier-duty self-healing talents). If things stay as they are, any death knight who's been at least casually running a few new heroic dungeons and maybe done a few Raid Finder runs in patch 4.3 should have little trouble making the transition to Mists of Pandaria from a survivability standpoint.
For my first leg of beta testing, I focused mostly on two-handed frost and blood leveling. Both play very, very similarly to their live versions. While we have lost some runic power generators, Might of the Frozen Wastes seems to do an admirable job of keeping two-handed frost in runic power, to the point where I still found myself in Unholy Presence to be able to spend it all. That said, I don't say this to complain; I like the two-handed frost playstyle. It gives me a good amount of stuff to do with juggling procs, runes, and runic power, and the ability to choose Runic Corruption over Runic Empowerment means I can eschew the one part of the randomness I truly disliked.
That said, I still do think the people who want Killing Machine's proc to be less random or to boost frost damage so it gives us more choices instead of nudging us to spam Obliterate may have a point.
The fight has only begun
Right now, I've been mostly focused on feeling out the leveling game, and at the moment, it looks pretty solid -- I will not lie. We haven't been changed too radically, but that may be a good thing, as anyone who's been on the death knight roller coaster for far too long can tell you. We can hold our own against mobs, we can survive relatively well, and our rotations remain much as they have been, so no time tripping up and relearning everything you knew about the class just to level.
There are still problems, of course, and there are bound to be more. There's some question over whether some of the new abilities such as Soul Reaper favor one spec over another due to their damage type. I would bet that a lot of abilities will need to be switched to shadowfrost or physical damage to keep the balance there.
There are also questions of our damage. Blood tanks especially are seeing low damage amounts. In addition, many old problems remain. The balance of dual-wield and two-handed frost is very much up in the air. Unholy still has the same old issues with AoE burst damage, damage ramp-up time and pet statistic balances (mainly the fact that mastery doesn't scale with pets). Frost's Rime and Killing Machine procs are still considered too random by many.
Whether Blizzard will fix or even address all these issues remains to be seen, but I can at least confirm that the base systems are solid. Casual death knights should have little problem making the transition, at least. The real tests will probably come later. Right now, the game is capped at level 86. Once we hit level 90 and maybe even start running the new raids, we'll probably get a better of idea of where all these talents and mechanics really lead us and how well they actually stack up.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.