I've quit WoW two times.
The first time it was because I didn't find any fun people to play with, I either partied with people and found them nice and fun only to notice that they played three times as much as me and the next time I saw them online they were 10 levels above me... or it was the opposite, that they played like once a week or so... Reached lv 42 at best during this first period.
The second time I had been playing with a static party and I finally got a max leveled character, a 70 belf hunter. Then when we all were lv 70 and none of us really wanted to raid it ended in a couple of alts and finally in us all moving over to LotRO.
With LotRO I payed for the game and a 60 day game time, played for about a month, found the game disastrously boring, regretted spending the money, uninstalled the crap and haven't thought back on it. That ended in me losing contact with the guys I played with though.
Until I managed to reach them again via the only one I knew somewhat irl and a few of us got together for a new run in WoW. Now I soon got a 70 Draenai Priest and then it's probably gonna end again... unless I give the new expansion a chance, which I guess is quite possible. But unless they add in some good 5 man end-game stuff (That yields epics) I'll probably leave again before or after hitting lv 80...
I've also played quite the heap of free to play MMOs as well as private server RO. In the case of RO it's usually been that I've reached the maximum level and can't be arsed to really do anything more in the game... especially on quick level, maxlv 255 servers, Killing MVPs solo with my assassin, slicing at the speed of light, was fun for like an hour xD So I quit. With the Free to play it's usually the "God this is crap!" factor that kicks in =P That or I get annoyed with the people who buy super equips from the in game store for real money. I know that these companies want some kind of income as well but I still hate finding out that the nice armor I sought after is only available for real money credit... Especially when the game is still in "Open Beta"... these games NEVER leave the damn beta... They're closed betas, that you still only have to sign up for and perhaps agree to some pointless NDA, for like a few months then they switch to Open Beta *Cheers all around* and stay that way for the rest of their existence...
Ok, rant is over now, move along, nothing to see here =P