The entire Shadow of the Beast series, it was too hard.
Police Quest 3, because you can't slow down the driving sequences, and I never had a pc slow enough to play it.
Morrowind, since I never had a clue what to do next; I kept getting lost.
The first Assassin's Creed, since all of a sudden the counter and deflect move didn't work anymore, and I didn't want to learn how to fight without those moves in the last mission.
Dungeon Keeper; there's a point in there where the missions get a *lot* harder all of a sudden.