Some of these replies have made me remember a few more of mine.
God of War. I played it all the way through, and loved pretty much every second of it. Then I got to the final boss. I just simply couldn't do it. I got slaughtered pitifully over and over again. Then suddenly one day my disc just broke. No scratches or anything, my PS2 just refused to read it. It wouldn't even spin. It still works with all my other games (apart from Gran Turismo 4), so I don't get what went wrong. But it means I never played any God of War game again, despite really enjoying that first one.
There's Morrowind too. A lot of people have mentioned various Elder Scrolls games, but I'm not sure they really qualify in plenty of cases. It takes forever to completely finish those games, and I don't it really matters how many or which quest lines you bother to do. Although the main quest line is probably more "important" than the others, it's still doesn't really count for much. You can even ignore them all and experience a huge chunk of the game just through random side-quests.
That said, with Morrowind I've barely ever got anywhere with it. I've started it a bunch of times, but then either screwed myself with a crappy build or just got bored of it and moved on. My most recent try has gotten me further than any other attempt, and I think I have a decent build for how I play, but I still haven't experienced the vast majority of the game. If I remember rightly I'm only like level 3 or something.
AroLombardi said:
Darksiders 2. The combat is fun enough and I like the outfit customization, but as soon as I got to the Land of the Dead I lost interest in going for another 2/3 or so of the game.
If it's any consolation, after the Land of the Dead the game narrows down dramatically. Although the dungeons themselves get larger, the number of them drops a whole lot and the overworlds become almost nothing more than a straight path from the Tree of Life to the one or two dungeons in the area. So it picks up the pace quite a lot, with the exception of one dungeon that is a bit of a drag.