I've been reading up a little on death-knights.

From what I can gather, Warriors will still be MTs most of the time. They seem to be getting an AoE damage ability so should make em more viable for boxing too
Paladins will always be AoE kings methinks just won't go any other way tbh
Druids are likely to remain very high AC and avoidance so great for most bosses (also generally high threat as OT)

Death-knights are as stated high dps(probably high tps for that reason), somewhat self healing, avoidance tanks, and seem likely to have a lot more spell absorption etc talents/abilities and this will set them apart. As a tank, although fully capable of tanking melee bosses and group pve, they will reign supreme vs caster mobs/bosses. Looks like resistance gear won't be an issue for them.. which makes me wonder how they will fair in PvP

oh well ^^ it's mostly still in development