Really it is a preference call.

I personally go with Paladin or Druid tanks, as I find them easier to tank based on the spam of fewer buttons.

Generally a Pally is my top choice.
But for teams which are predominantly casters, especially PvP teams, I like that a Druid can both be a Bear tank and a Balance (ranged) DPS.

From what I've heard of beta, DK's have been the top tank so far.
Warriors are superb tanks, if you micro them; click this ability in this situation, click that in another -- if you can manage that (procs) they're a good choice too.

Encounters are balanced against the average group of players.
And boxers are generally better players, as you can be sure your DPS won't stand in puddles of acid or the like.
So whatever you go with, will likely work.
We need to learn the mechanic, such as how to move the slaves independently of the main.
But once we do, no one in our groups (vs half a pug) make stupid mistakes.