I find this idea interesting. Quitting a game because it's bad is a no brainer. What about those games you never have finished even though you are enjoying them? I can think of three: STALKER:SoC, KOTOR, and Sid's Pirates (the remake). All I have put alot of time into, all were fun enough, and yet all spend a lot of time sitting there doing nothing.
I think part of it is at least when faced with serious commitment factor, if I stop playing the game, I lose all my game momentum. It's heck to get sucked back into the game again. Or worse, I *know* the game is going to consume alot of time, so I keep putting if off because I'll have more time later.
Of course, my current biggest game shame is my huge collection of unfinished DOS games. I have a copy of Full Throttle still in the shrinkwrap that I personally bought new full price. I wish afford to be that young and stupid again.