Assassins Creed. Played the first game but it really burned me with all its crap such as unskippable lengthy super-boring cutscenes and really crappy enemy-AI. Also every trailer I have seen for subsequent AC-games, Ubisoft really does not seem to understand the concept of Assassins. In both the brotherhood-trailer and the AC3-trailer, the main character does a strange frontal attack on a small/very large army. That isn't really how I picture super-cool secret society of assassins to work.
Half-life 2. I loved the first game, played it 5-6 times... But despite trying several times to like half-life 2 I just cant. Level-design is horrific so I often lose track of where I am supposed to go which disturbs the flow or even kills me when enemies are shooting from above. I am never having fun with the game, and the story is not as good as in the first game.
Original X-com. MAN I tried! I never really played the x-com games when I was younger, I dont know why, just never had the opportunity. But I love the kind of games that X-com is so I thought I would remedy that now... but MAN it is one hard game. I am usually good at games, but despite playing on beginner and looking through some tips&tricks guides I still get my ass handed to me, especially in alien bases. Maybe I have just gone soft over the years. Looking forward the remake though!
Majoras Mask: I LOVE Zelda, but this is the only regular Zelda (except 2) that I haven't finished... Not because it is hard or because I dont like it, I just HATE having a time-limit that stresses me out when I play. I did get quite far a few years back, but then I stopped playing and that was it...
Oh also, all bethesda-games. I tried with both Oblivion, Skyrim and Fallout 3 (I love post-apocalyptic settings) ... but there is just too much freedom. I want the game to take me by the hand and say: "come here, this is the way to go" while giving me a great story as a reason to stick around. As soon as I get out of the prologue in bethesda-games and realize I can go ANYWHERE, I just loose all motivation to play.
Guess thats it for now...