Well you’re on the right track since Warriors are by far the best tanks. I actually wonder if there will be too many warrior tanks!
Everyone knows the overpowered classes for Classic:
Prot Warrior
Hunters
Frost Mages
Warlocks
Rogues
Healers may be a bit more open. I’m pretty sure both Priests and Holy Pallies are good healers, but I haven’t seen anything about Shamen and Druids.
That’s pretty much it. Other classes/specs are viable, but people will gravitate towards the overpowered classes, especially considering that they won’t ever be re-balanced.