The death of diseases
If you play an unholy death knight, you'll probably notice something relatively quickly. You're missing Ebon Plaguebringer's magic debuff. Of course, it's disappointing enough that you lost some group utility, but where things really hurt is that you no longer have that third disease. Since the third disease isn't there bolstering your other diseases and giving one more disease for your Scourge Strike to feed off, it's a pretty decent chunk of personal DPS lost as well.
Luckily, Blizzard did recently buff Scourge Strike a little bit to compensate, making the shadow damage portion hit for 25% more damage per disease. This does close the gap quite a bit. At the same time, it's one more sad little thing about the changes this expansion. Early Wrath-era unholy, to me, was the pinnacle of the class, at least in terms of style. As unholy, I felt like a proper sinister mage knight, with a permanent ghoul, a deadly aura in the form of the original Unholy Blight, four diseases sapping away at my enemy's strength, and Bone Shield giving me an even more sinister and brutal appearance.
Alas, most of that is gone now, and with the third disease officially flung out the window, I still feel like unholy's a shadow of its former self. The damage buff fixes the damage issue, in theory, but I wonder if with all these changes that unholy hasn't lost the soul of what it used to be.
Get your blood boiling
One thing that death knight tanks have been concerned about for some time is the need to trade off threat and debuffs for extra mitigation in a way other tanks really don't. Since the runes to refresh (or apply, if Outbreak is on cooldown) Weakened Blows via Blood Plague are also used for Death Strike, we take away from our survivability, our raison d'etre to be a tank no matter what we choose to use our runes for. We did get a 30-second cooldown on Outbreak in late Cataclysm, which has proven a useful stopgap measure, but Mists beta came around and that cooldown reduction was gone.
In a recent build, Blizzard's provided a new alternative. While you could always talent your Blood Boil to spread diseases, blood death knights will get the ability to spread diseases by default as part of Scarlet Fever. This means that once you use your initial outbreak, a Blood Boil will allow you to spread your diseases by using a blood rune that you can't use for Death Strike anyway.
Another change for Blood Boil helps out unholy death knights. Blood Boil is now on Reaping, which means that unholy death knights should find their AoE rotation a little smoother, with less need to use Icy Touch on orphan frost runes. You'll still need to do a round of single-target DPS to set up the death runes, but once you do, the unholy AoE rotation will smooth out quite a bit. Still, A lot of death knights are pushing for a removal of that Festering Strike ramp-up altogether, and I can't quite blame them. That said, as long as AoE encounters don't end so fast that an unholy death knight is stuck with 4 fresh death runes and nothing to use them on, it should at least be a marked improvement.
Take good care of your ghoul
Permanent ghouls have had an energy problem for some time now. They use their energy for Claw, Leap, and Gnaw. Leap and Gnaw are excellent gap closers and crowd control, while Claw boosts a ghoul's DPS by a decent amount. Unfortunately, if Claw's on auto-cast, you can find your ghoul is unable to use Leap and Gnaw when you need them most. This means that an unholy death knight who wants to get the most out of his ghoul, especially in PvP, needs to keep Claw off auto-cast and on a macro, and monitor its energy to see when it's safe to cast Claw while leaving enough energy behind for Gnaw and Leap.
With Mists of Pandaria, this should be less of an issue, since Ghouls will refrain from using claw unless they have at least 60 energy. This will leave them enough extra energy saved up that you should have those extra skills available when you need them. You do have them on a macro, don't you?
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.