Well...I'll try not to spoil it. Let's just say when you do the right quests, you get sent to the north west part of the zone, where Alliance and Horde both are getting ready to attack Arthas (at the Wrathgate). You do a couple more short quests, which unlocks a cinematic, which unlocks an epic battle, where you fight alongside Thrall, Sylvanas, and Vol'jin. That's about as far as I'll go...but you don't want to miss it.