I was 5-boxing a multi-class group back in TBC and then retired for a while. I recently reactivated two accounts, and have been having tons of fun running them through quests towards level 80.

I mention this in case anyone out there is thinking of multiboxing but hasn't yet taken the plunge. 5 accounts can be daunting at times, but running with only two toons is practically the same as running with only one, and then you have two characters to choose from at level cap. As others have mentioned, another advantage of running 2 or more is that you can finish those more difficult quests where groups of 2-3 are normally required.