1. DKs have lots of new cosmetic options. One of the things that first drew me to death knights, even before WoW implemented them, was the look and feel. I loved the idea of being a dark hero dispensing a brand of infernal justice, and the death knight class delivered in spades. I had Unholy Aura, Unholy Blight, Bone Shield, and a ghoulish squire by my side to declare that I was the boss, the king, the lord of darkness.
Arguably, the Cataclysm revamp sort of lost that a bit, but it's all coming back in spades in Mists. Unholy Blight is back in a modified form. The level 90 talents, while admittedly lackluster for actual PvE purposes, allow us to desecrate the ground or summon howling snowstorms or draw all our enemies inward. Our minor glyphs also now include some pretty fun cosmetic changes. Your ghoul can finally, after all these years, look like a geist. I've wanted that since I first saw them slinking around Acherus, and now, they shall be mine.
Sure, maybe we won't be the overpowered class this expansion, but we'll be the ones who look the coolest.
2. Two-handed frost DPS is still good. Two-handed frost has, of course, always had issues. With dual-wield frost having a lot of innate advantages, two-handed frost has had to take a back seat. This may not actually be the case in Mists. Two-handed frost has, surprisingly enough, managed to come through the changes relatively unchanged. This is because Might of the Frozen Wastes provides enough extra runic power that you can stay in Unholy Presence, thus keeping the relatively fast-paced speed while providing enough resources to prevent a drought.
Mind you, the playstyle's not completely unscathed. Plague Leech and Blood Tap are still optimal talents for DPS that will add some extra complication to the rotation, but for the most part, you'll be clear to keep on trucking like you have before. If, like me, two-handed frost was your favorite PvE DPS method, you could be sitting pretty in Mists.
3. Blood tanking is still viable. Blood tanking had its issues during Cataclysm, but what you should remember is that even at their worst, the issues with blood tanking only got horribly crippling at the heroic raiding level. Mind you, that doesn't mean they weren't serious or shouldn't be fixed. What it does mean, though, is that at the dungeon or Raid Finder level, being able to spec blood and hop into a group is still a very, very nice ability to have, and blood death knights can still do that with a minimum of trouble. In addition, scenarios and challenge modes mean Mists of Pandaria will have more end-game small group content than ever before, and we will need tanks to run them.
4. DKs remain the soloing kings. It's a simple fact of life that some classes are better are soloing than others. One of the better classes is the death knight. A properly specced blood death knight can out-heal most old content. Certainly, not everyone's going to be like Mione, but with the level 90 level cap, who knows? Maybe you can solo the Lich King too.
This may not seem like a huge advantage, since you still won't be able to solo new content (at least not for a few more tiers), but keep in mind that transmogrification has sort of revolutionized old content. When you're putting together that perfect transmogrification outfit and you notice that there's this pair of gloves that drops in an old dungeon that would just be perfect, the ability to simply jaunt over to that dungeon and solo the drop yourself shouldn't be underestimated.
5. DKs are still not horrible at PvP. PvP has certain had its ups and downs in Cataclysm. I'm personally still pretty sore over the Hungering Cold nerf (and subsequent removal of the skill in Mists). That said, if you enjoyed death knight PvP in Cataclysm, chances are you'll enjoy it in Mists. Most of the stuff that's bothering death knights in the Mists beta is PvE-based, and the level 90 tier thus far is probably going to be more useful for PvP than PvE.
6. The beta isn't over yet! I saved the best reason to play a DK in Mists for the last. The death knight class is still in beta. The state of the death knight class is troubling to many, but we still may have time to fix it. With level 90 unlocked, we can start to take some real tests and run some real numbers against end game content, with our entire skill set ready to go. If you are concerned about the state of the death knight class, I urge you to do this, then post the results to the beta class forums. The more feedback we submit, the bigger chance we have to see changes.
This tactics isn't foolproof, mind you, but more evidence is good. And if we don't see the changes, well, at least we'll be able to say I told you so when Blizzard has to patch issues later.
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.