What keeps me doing it is that I have so much invested in all my characters. All that work leveling up all those toons etc etc... When the game becomes stale I stop until it gets more things that interest me.

Right now I'm bored as hell with WOW/Multiboxing the whole deal. Last month I shouldn't have renewed my accounts but did like a retard and have logged in like 4 times all month. So I'm cancelling my 4 accounts until Brewfest because I need to farm the kodo/ram, and for Halloween headless horseman's mount. Kinda sucks how they are spread out where I have to drop another 150$ before Cataclysm. But after those events i'm out till Cat which shouldn't be far after that.

Really though i'm so burnt and bored with WOW that i'm playing City of Heroes for the first time and enjoying it. Also Mafia 2 finished it and will play through again it was so amazing. Just get away from WOW play some other games and WOW will be fun again.

To answer your question overall. Playing one character is boring in light of what you can do multiboxing. Going in a dungeon with 4 other people I feel as if they hold me back and it's less of a challenge but more frustrating. When I do a dungeon by myself with my team I feel that sense of accomplishment.