I played a Warhammer trial a while ago and there are some nifty concepts I would like to see implemented in WoW too.
One was, that tanks cold build up a massive damage potential if they were ignored in PvP and the healers they protected were attacked instead. That's a nice way to give tanks a PvP role I think.
But since this has nothing to do with this thread, here's what else I thought of as a goo idea:
You get automatic stat improvements by speccing deeper into a given tree. So say you would put 5 Points in a defensive tree so you would get a 20% boost to your armor. This example is completely made up I must confess, as it's been a while since I played WAR and I only played it briefly. But I think that was the general idea and a nice way to factor these passive bonuses in.
In WoW it could look somewhat like this:
1) Every talent is assigned to one of the roles: tanking / healing / dps
2) For every x points in the given role group you get a passive bonus
So there would be a minimum of points you would have to spend into a talent group to have a decent performance, but what (of the active and funny talents) you choose is completely up to you.
Given this is pretty much simplified and requires a lot of work from people knowing much more about game design than me...