Nice Thread!
I think you could break this down into catagories such as I didn't finish the game becuase...
1. It was too hard,
2. It sucked! (or didn't like it)
3. It was boring or too easy.
I've beat a lot of the hard NES SNES games back in the day, but one that I could never beat though I tried many times was X-Men Arcades Revenge for the SNES.
I Never finished FF VIII, it is the only FF game I played and didn't like (though I must admit that the last one I played was X)
I didn't finish Billy Hatcher and the Giant Egg because the gameplay was boring.
Those are just I few I can think of right now. I usually finish games that I buy/play. Not necessarily 100% (though I will do that if I like the game enough), but beat the game.