World of Warcraft
I started playing not long after it first came, already being a fan of the Warcraft franchise, and continued playing on/off right up until before Wrath of the Lich King came out. I remember playing Warcraft 3 once online, thinking to myself, when will I stop playing this? ... the answer was World of Warcraft, and it's safe to say it wasn't helping.
at first play, the game is fun at first. it's simple to pick up and play, it's colourful, the landscapes are vast and look good and you can see and interact with all the other players around you, which is novel. unfortunatly, when the glamour fades off, you notice it's better to play with friends, usually on things like Skype or the in-game voice chat. I guess it helps with playing the game, like most MMO's it's a grind game, and grinding without talking, chatting or listening to music can be hell.
World of Warcraft stopped being fun for me when I reached 80. that's the level cap, no more experience to collect, no more levelling, so, you're done. however, MMO's are limitless (so to speak) and apparently "endless" (which is bad marketing scheme, milking the game to death until a new expansion, and dragging the game on more than it should be) - what's waiting for you at 80? end-game instances & raids.
from a personal point of view, the repetition of them that has been copied out through quite a few instances is boring, same strategy and instance boss that has been copied from the original World of Warcraft to now. if you want to raid, your only good chance of raiding is finding a guild that handles PvE content, which can be a bitch. trying to find a good friendly active raiding guild can be stressful, well, from what I remember. and usually before your 'accepted' into most raid groups, you need fairly good gear to start with, which you get from end-game instances. now, why should I grind an instance one-hundred times or more just to get even more better gear from a raid? - I'd still have to grind raids either way, it's from one grind to another. atleast when your levelling it's just one big grind to the end, but ultimately, the end sucks. oh wait, there is no end in an MMO.
apart from all this, the last thing you can do for fun is PvP (which is broken, classes are in need of fixing for PvP to work properly) fighting players from the opposing faction in Battlegrounds or Arenas. even this turned into a grindfest, intended to make you play for longer and longer stretches of time, since if you want to get anywhere you need good gear. now let me ask you a question - what's the point in spending all of your time grinding at 80 just to get slightly better, and I do mean slightly. it's depressing and repetitive. grinding should never be the main part of a game, it's not user-friendly at all.
do I play World of Warcraft anymore? - no. the only thing I'm looking forward to from Blizzard is Diablo III and Starcraft II. I'm positive these games will be awesome, but who knows? we'll have to wait and see.