As you know if you've read Sacco's excellent (and completely spoilerrific) guides to the Coliseum raid and the Tournament 5-man, there is a tacked-on reason we're doing all of this stuff, and it's that the Horde and Alliance want to send their strongest people to go after Arthas, and the Tournament is a way of sussing out who's most worthy. But though that makes for some great lore cutscenes, doesn't it still undermine Arthas' strength? How powerful can the guy be if there are so many people lining up to kill him we have to fight over it?
Don't get me wrong, we're definitely looking forward to the Coliseum, and it's definitely brought some interesting things and rewards to the game. Just like CoT, in the light of the lore and the MMO genre at large, it's a great addition. But it is funny that the great Lich King's main tactic seems to be to have us fight it out between ourselves way before we ever bother taking a shot at him.