The immediate problems
When doing some preliminary fighting and questing as an unholy death knight in Jade Forest, I noticed two issues that really nagged at me.
The first issue has to do with runic power. As you know, Blizzard removed a lot of talents that helped our runic power generation without really replacing them or adjusting anything else to help. Unholy's most obvious loss is the old version of Runic Corruption, which cheapened the casting cost of Death Coil. If nothing else, we could use a cheaper Death Coil as a base feature of the spec.
AoE has also taken a nose dive. This has been an unfortunate issue of the unholy spec all through Cataclysm, but its only gotten worse in Mists of Pandaria. There are a few reasons for this. One is that Death and Decay got a huge nerf (mostly via the loss of Morbidity), likely to keep it out of unholy's single-target rotation. In addition, diseases once again spread at 100% damage for all specs. In theory, neither of these changes were a bad idea. Not having to cast Death and Decay to reach maximum single-target DPS is a good thing, and frost and blood knights being able to use Pestilence and other abilities to get full diseases up means combat is smoother and more fun.
Unfortunately, unholy has not received any compensation for the taking away of two important AoE tools. I actually don't mind either nerf per se (although I would have preferred it if unholy was buffed so it didn't need Death and Decay in its single-target rotation), and it's unlikely they'll be rolled back, since both do solve some long-standing death knight issues. Therefore, unholy really does need more help. It's possible that Desecrated Ground, when combined with Dreadblade mastery, could do it, but what I'd really like to see is for Wandering Plague to make a comeback. That was one of the more iconic old abilities of our tree. It dovetailed nicely with unholy's focus on diseases, and it could give critical strike rating some much-needed value in the unholy tree.
Festering Strike: A failed experiment?
One other issue that has bugged me since Cataclysm continues to bug me in the Mists of Pandaria beta. Making Scourge Strike cost one unholy rune may have seemed thematically sensible, but it continues to make unholy the awkward stepchild of the class. The other two trees are built around the frost/unholy strike, which means unholy requires an extra level of work to switch over to, and cross-tree abilities are that much more awkward to activate. This is especially apparent with Death Strike, a staple of death knight leveling and survivability that, while nerfed pretty hard, still has a place. With unholy death knights, that place involves completely throwing off our rotation.
Putting Scourge Strike back to a frost/unholy setup would not only make unholy's rotation flow better when it's time to add in diseases or heal up a bit, it would make switching between frost, blood, and unholy more natural and intuitive for all death knights, encouraging more variety and experimentation. You could then take the disease-extended powers of Festering Strike and make them an unholy-only addition to Blood Strike, thus taking away a somewhat confusing extra strike from the death knight spellbook in the process.
Now, all this isn't to say that unholy is completely worthless for leveling. I was still able to fight one-on-one relatively easily, and Death Pact made healing up a snap. For the most part, you can play the tree much how you play it on live, you just have to be aware of a couple of new weaknesses -- and be aware that, yes, it has many of the same weaknesses it has on live servers still.
You probably won't notice the pet problems too much while leveling, but they're still there, and they still threaten to be a pretty big problem once we get to dungeon running and endgame raiding. By now, you probably know the litany. Pets still do not stack with mastery, leaving us in the awkward position of gathering an important stat that doesn't actually do anything for a huge part of our DPS. The answer for the Gargoyle seems clear enough. Make it do shadow damage instead of nature damage, and let that shadow damage be affected by Dreadblade. For the Ghoul, it's less clear. Perhaps the Ghoul just plain needs to get its own unique effect from mastery completely separate from the shadow damage boost of Dreadblade, even if it's just a straight-up damage percentage boost.
Ramping up is still going to be a problem for unholy as well, which not only makes it a more annoying leveling tree but continues to give endgame DPS unique problems as well. Dark Transformation and Shadow Infusion, at this point, do nothing but complicate the rotation and make deciding when to cast Death Coil more complicated than it should be. Complication isn't a bad thing, per se, but in this case, it's adding an extra layer of frustration by forcing unholy knights to hurry up and wait as they try to stretch out their Dark Transformation uptime, and it's making unholy DPS a bit more streaky than it should be. That mechanic feels like it needs a redesign to be less streaky, or unholy needs a more general buff to be able to summon burst DPS on demand and allow a Ghoul's DPS potential not to be tied to one specific, very finicky cooldown.
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.