The first MMO I used to play was Ultima Online: played it for some years then I quitted it 'cause I had no more interest in it.
Later after that, in 2005, one year after WoW was released, I started playing it. The same thing that happened to Ultima happened with WoW, though: I quitted it almost a year ago, so making the calculations, I played it for 9 years. I was pretty good at the game, had a decent rating in PvP and stuff like that. My mains were a rogue (undead, which had maxed engineering/mining and then eng./herbalism to get some golds out of the professions), a Warlock (an undead, same as before, though he had alchemy/herb. - I needed a lot of golds/materials to do the Mechano-Hog, which is probably my favourite mount in the game for its originality) and at last a shaman, an orc, that had leatherworking/skinning just for fun.
I had alot of mounts while I played the game, and maybe achievements, raids, the guild and mounts are the things that brought me to play this game until a year ago.
Another MMO that I played was Warhammer Online - Age of Reckoning!
There I had a 30-35 rank Slayer. I continued to play the game until it closed. I played even Aion, when it first came out, and I got really good there too, I used to play it 4-6 hours per day while I couldn't go out because I felt sick for quite alot of time. Eventually after a week of doing so and after some time that I spent costantly on the game (2 hours per day), while I wasn't sick aymore, I managed to get to end-game pretty fast (considering that I played it alot, it's possible).
I even played League of Legends, I got to Platinum V then because of my dead PC, I dropped it.