I started multiboxing ghetto-style in Dark Age of Camelot, which was tons of fun, but I had no idea back then just how gimped that game was in terms of follow/assist mechanics. I gave it up for about 6 months when WoW came out. I think I decided to start multiboxing WoW about time my shaman was dying repeatedly in Un'Goro Crater. I've been playing WoW off and on for most of the time since as it's really the most solid MMO available in all ways.

Tried EVE for a while; not bad but just too much calendar time to get to where I wanted to go skill-wise. I had tons of money but couldn't fly what I wanted without another 6 months of AFK skill training.

Tried Rift for a bit with some friends and family and while we liked the game's leveling mechanics, it had a sort of...unorganic feel to it. Like the world just wasn't alive enough. That, and hunter pets wouldn't tank for shit. Made it to end game and started getting bitten by the "trap" mechanics that purported to be difficulty.

Tried Diablo for a bit. Fun game for an orthogonal arcade game, though it got old after the second run-through.

Dabbled in a few others but honestly, WoW keeps me coming back because it's the only game where I can just login and blow stuff up quick, fast, and in a hurry and not feel like I'm Working at it. The UI, playability, and feel of the game are just excellent. Yeah, I get bored of the content like anyone else but then again, Tetris is pretty freaking short on content and it's one of the most popular games of all time.