Divine Divinity. I've tried to play it four times now, I think. The first time I got all the way to the second map, but I didn't do a whole lot on the first. I got bored and quit. Second time I didn't get out of the catacombs. Same with the third. The fourth I did quite a bit in the first map, but I didn't get far in the second before I just got fed up with it again. I couldn't really enjoy it.
Castlevania 64. I haven't played the game in a couple years, but I played it almost religiously when I was (much) younger. It took me forever to progress through the game, though some of it had to do with the Nintendo 64 being at my grandparents' house. Still, I would get all the way to that last boss, which is actually three bosses, each one harder than the last. I finally got to where I could pretty consistently get to the third boss, but that thing would kill me quite quickly, which meant I had to start at the first boss.
Fallout. The first time I played it I got to a point where I wasn't really quite sure what to do, so losing interest fast, I just quit. The second time I played it I got really far, but I got to a point where I was doing stuff I wasn't sure I was supposed to do. Again, I kind of lost interest.
Morrowind. While I know completing this game would really take a long, long time, I included it because, well, I've never even gotten close to completing the main quest. The longest I have ever been able to stick with the game is a little over a week, and that was when I first got it and I kept trying to convince myself that at some point I was going to see this glorious piece of RPG greatness. I never did. Not that I think the game is good, it's just not for me. Everything moves to slow, animations are bleh, and combat...
I hate the combat.