I think this is more of a two-part question: Are there game's that you have flat out just not finished (and have no intent to), AND what percentage of your games have you currently completed?
...if that makes sense...
Personally, probably about 10% of my games, I really have no desire to return to. Of all the games I have, I've completed about 40 or 50 percent. It's a sad, sad, reflection on the fact that I have less time (and more games) than I used to when I was younger.
It's somewhat of a curse, really. Work means you can't game as much, yet some disposable income means that you can get your hands on the games (which you consequently can't play). Vicious cycle, right?