The guys at Ten Ton Hammer have put together an editorial entitled "Breaking the Holy Trinity Mold" that explores how party configurations in Dungeons & Dragons Online: Stormreach will differ from other MMORPGs. Check it out:
Even the simplest of roles in the holy trinity, the tank, gets confusing in D&D. Traditionally, the holy trinity philosophy preaches that the tank is a damage taker: a class who can take a pounding and keep a monster's attention away from other group members. In D&D terms the best tank is obviously the fighter. Or is it? The Barbarian has more hit points, and damage reduction to boot, yet is impaired by not being able to wear heavy armor. Then there's the Paladin. He has healing spells, lay on hands (a form of heal), good hit points and can wear plate armor and wield that great big two handed sword. What of his tanking abilities? Multi-classing brings in more confusion and complexity. For instance: fighter/barbarian an odd combination, but build it right and you can have a pure animal on the battle field.