I always did pretty well back in the day with a fury warrior (me) and a ret pally (guildy/friend). We tore through pretty much everything.
Actually, I tend to be pretty one dimensional and stick to a single class as my main. My friend, on the other hand, likes to alt hop and even do crazy things such as get a character to 90 then delete it to level one again. So, seeing as how we play together a lot, I've ended up running around/questing with pretty much every class he's played. From what I've found, we tend to do better when he's either on one of his plate DPS/tank classes or any class that can heal me.
Two Paladins. The fact they're a hybrid means you can do any spec combo you like. Even if both are ret you still have the ability to heal each other should things go bad. In general though I think it's their diversity.