My multiboxing goal was first to level up one character of each class to solo raid and PvP with. I thought a mixed team would be a pain to play, but it worked out far better than I had feared.

At the moment I play a paladin (protection), warrior (arms), paladin (retribution), mage (fire) and shaman (restoration) team. Five different classes (OK then, four different classes but five different specs) with two melee DPS isn't the easiest group composition, but it's perfectly doable once you've got the hang of it.

The group is level 65-68 now. I have cleared all instances up to Mana Tombs where I still wipe on the last boss. A warrior with bladestorm, a paladin with divine storm and a windfury totem really is the shit! And repentance is a great CC spell (even better than polymorph since it's an instant cast and has a longer duration for PvE).

I also have a warlock, another mage, a feral druid and a rogue that I sometimes throw into the mix, but they have fallen behind level-wise (they are only level 60-61).