I haven't played a Death Knight on the live realms yet (I leveled one through the starting experience during the beta, and am only 79 so far on my main), but I have grouped with quite a few of them now, and the ability that real stands out to me and others seems to be Death Grip. A lot of the other Death Knight abilities are just new versions of other classes' spells, but Death Grip is a pretty new mechanic -- instead of charging or jumping away from a mob, you're bringing the mob to you. And with all new mechanics, players have found new ways to play with them. As you can see in the video above, Death Grip, when chained by a few Death Knights, can even be used as crowd control.
I've seen it used in a few other wild ways, too -- it works great as an interrupt, and when combined with a Hunter trap, it's finally a reliable way to trap ranged attackers and casters. And most of the Death Knights I've seen use it for pulling -- they suck the caster in from a group, and the rest of the mobs come with, and group right up for AoE. And I haven't even been to any PvP matches with Death Knights yet -- I imagine the uses there are even more hilarious, not to mention that I'd be yelling "Get over here!" every time I hit it. Very fun mechanic for the new Hero class.