The biggest benefit comes with a dungeon that is sure to be popular: heroic Scholomance. We can probably assume at this point that it's still going to be populated by undead, which means that any death knight coming into there is going to have one more awesome tool in his kit.
Anyone who's played a warlock or shadow priest can probably tell you how useful controlling undead is. Being able to control a member of a pack of mobs isn't just a method of crowd control -- it's a extra level of advantage over the enemy, an extra level of strategy.
If you feel like the pack's too large, you can send the mob in first and let the others kill it (assuming, of course, Control Undead aggro works similar to Mind Control aggro). If a pack includes varieties of mobs, you can choose, say, a healer or a caster with an awesome AoE spell. This opens a whole new level of fun and usefulness for a death knight, even if it's only in certain specific dungeons.
Of course, it does have its possible downsides. A mind-controlled creature breaking at the wrong moment can cause chaos, and pathing errors can bring an entire dungeon on your head. Luckily, we're more or less used to the latter thanks to our ghouls, but it's still something to watch out for.
Remorseless Winter, one of our top-tier talents, is still probably most amazing because it's basically a Lich King-level boss move adapted for the death knight class. You want to feel like a hero class; feeling like the Lich King himself is probably a good way to go about it.
It's also a skill that could have a lot of applications. Not only does the area of effect slow enemy speed, but if they take five ticks (out of eight) of the ability, they're stunned for 6 seconds.
I'm sure your head has been spinning with possibilities over this for a while now. For tanks, as long as you can get good initial aggro (Hello Death and Decay), it's a pretty good guarantee you'll have a lock on basic trash packs, especially since a 1-minute cooldown probably means it'll be refreshed by the time you start your next pull. Even if you lose aggro, the slow and stun means that the mob will very easy for a hapless healer or ranged DPSer to dodge until you can pull it back in.
For DPS, it's a little iffier. Remorseless Winter, in a lot ways, has the possibility of becoming the Army of the Dead of the Mists expansion. Just imagine this: A tank is try to set up a pull. To avoid patrols, he wants to set up the mobs in certain area. A couple of them are casters, so he throws out a silence on one, steps around a corner so the rest will follow ... and then the DPS death knight runs in and hits Remorseless Winter. Suddenly, all the mobs are walking slowly and half of them are now aggroing the DPS, and just as the tank's rushed in and gotten them all under control, they fall stunned, just in time for the patrol to return and wipe the whole group.
That said, I think the advantages far outweigh the chance of a couple of ditzes doing some bad stuff with it, so I'm hopeful we'll get it more or less intact unless beta testing absolutely reveals it to be overpowered. It's one of those tools that, more than being useful, will define our class' flavor in the process.
Diseases and debuffs
Diseases aren't technically new to us. They've been a cornerstone of the class since the beginning and, in my mind, one of our more compelling mechanics. But they've also had a rocky relationship with the class, being constantly buffed and nerfed due to scaling issues, sometimes to the point that diseaseless rotations became in vogue during certain patches.
In Cataclysm, diseases are coming back with a vengeance. Not only is Outbreak back as a skill (which should be a huge relief to blood and unholy death knights especially), but the first tier of the new talent tree is all about giving us even more ways to spread our diseases. Since the vanilla way of spreading diseases via Plague Strike and Icy Touch has become more and more inconvenient, having newer and more exciting ways to get off the ground with our rotation is a great thing.
One thing some people seem to forget about, though, is the concept of the debuff. Diseases don't just benefit you; they benefit your whole party. The Brittle Bones debuff that frost death knights get, for example, can be instrumental for success in a melee-heavy group, and in Cataclysm, unholy death knights will be getting their own version of the debuff (at the cost, alas, of Ebon Plaguebringer's magic debuff). Frost Fever slows enemy attacks in and of itself, and blood death knights can add another weakening affect via Scarlet Fever, which they'll get as a spec-specific skill in Mists.
One of my biggest problems with diseaseless rotations has always been that they seem to take away a big chunk of the class flavor by eschewing diseases, but there is another reason to avoid them. Those debuffs can be just the thing in the clutch. If you're serious about being an asset to your group, be sure to keep those diseases up. Just be careful that you don't put them on a crowd-controlled mob.
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.