WoW - it took a couple of years and a couple of false starts, but I eventually succumbed to the lure of Azeroth around 6 months ago. In my particular case, the lure was to run around as a big minotaur-lookin dude screaming "Woo Woo" at people. He's Level 40 now, bless him. I have other characters, but due to the fickle nature of my guildmates there's not much prospect of doing any raids in the near future. And I rarely team with anyone I dont know in RL.
EVE - Despite all the recent controversy, I've actually returned to the game after almost a year away. This time round, my one and only character is being outfitted as a serious miner, with the ultimate goal being one of those new Tech 2 Exhumers brought in with the Revelations expansion. I play EVE solo, mostly with my attention elsewhere such as a book or IM conversation when no one else I know is gaming. I have never managed to convince anyone that this can be fun.
Guild Wars - This was my main game for 18 months straight, but boredom set in around Xmas time. However, one of GW's great strengths is the fact that my account is still there, waiting for me to pick up and play at a moments notice. I do worry that Eye Of The North and the sequel on the horizon are not going to be all they could have been - little tell tale signs like Dwarves and Tengu not being playable races, which I thought would have been a no-brainer.
Second Life - I was a paying Resident for a while, but I sold my land and went back to being just a wanderer. The main draw for me was the International Spaceflight Museum, and more recently, the efforts of NASA and other space-related organisations trying to get a virtual community up and running within SL.