But why? Are they referring only to single target, or heroic mode raids? My DK is Frost/Blood and I've played a Warrior and a Druid as well and tanked them while leveling up.
IMO, DKs are easily the most convenient tanking class around. Our AoE has no limits and we have a crapton of defensive cooldowns (which I often love to pop and proceed to pull an entire dungeon room without a healer present and live to tell the tale).
So what is it? I haven't tanked on a Paladin since wrath.