DK viability for a PvE tank is just as good as a pally. Probably requires slightly more micromanagement but not that much more. If you spec unholy thats where its more like pally tanking. A consecration-like ability, plus an AoE disease transfering ability, plus a talent that makes your diseases autospread makes AoE threat management really easy. You also have a pet that actually can take some decent hits, and has a built in stun. Plus you have a bunch of oh-shit buttons (bone shield, icy fortitude or whatever its called, etc). I also feel that the DK actual has far more viability in PvP than the paladin. The utility of some of the abilities (anti-magic zone for AoE protection), death grip, gnaw, along with some better ranged abilities like the ranged ice move and death coil make for more usefulness in PvP IMO.

Plus, they look infinitely cooler.