A pally as your tank is a lot easier to box with then others.
But, any tank class you want will work.
And until heroics, you might not need a tank at all.

I've found a priest to be the best healer, discipline or holy (although I prefer Disc).
Again, any healer type works, but a druid or priest seems better.
They have mobility with their healing and good AoE heals.
A shaman works too, and if you don't have a shaman in the dps, they're probably the top pick.

For a caster group, you'd want one of: Unholy DeathKnight, Balance Druid, Warlock for the 13% spell power buff, which is huge. Of these, a Boomkin needs to manage Eclipse procs for optimal dps.

A lot of people are experimenting with all melee groups.
Both for PvE and PvP, but you need some form of heals.
Pallies can self heal somewhat, other 5x the same class aren't likely possible.