My list:
- Grand Theft Auto: Vice City (PS2). I'm not too far away from the ending, I think I've got a couple of businesses to take over and then just a few storyline missions left. I'm stuck at the moment there, I can't remember the mission I'm up to but basically I fail it a few times, get frustrated and put the game aside for a while.
- Grand Theft Auto: San Andreas (PS2). I've actually never made it past the first area of San Andreas. It's a great game though I hate having to keep track of territory. Again there's a mission that's always caused me some grief and it's repeatedly been put aside in frustration. I expect I'll go back to it in the not too distant future though, I've got the PC version as well (same deal with Vice City) so if anyone wants to share a save file...
- Grand Theft Auto: Vice City Stories (PS2). Stuck on the mission where you meet up with Lance, the one that starts out as a rail shooter then you're back on foot for a shootout surrounded by gangsters. And everything's going well until Lance runs out and gets gunned down. Sometimes by you. Ugh.
- Super Mario Bros (the original, NES). Have played it a lot, but have never actually gotten up to World 8 and defeated the final/real Bowser.
- Super Mario Bros 2 (NES). I don't know if I ever played this one past World 4 or so. I wasn't as big a fan of SMB2, SMB3 is definitely my favourite from that era.
- Mario 64 (N64). When it came to the 64, I got into the Zelda games more than I did Mario 64, though I don't disagree that it was a great game.
- Grand Theft Auto IV (Xbox 360). Haven't touched this one in a while, as I've said in the GTA IV thread I just didn't get into it as much as I hoped I would. At some point I'll pick it up again but I'll probably go back to one of the last gen GTAs before that. I can't remember which mission I'm up to but I'm not very far along in the game.
- Assassin's Creed (Xbox 360). Picked this one up cheap last year and I do like it, but I have to agree with the common assertion that it's repetitive and that's what's holding me back from getting any further in it. It's also what's holding me back from picking up Assassin's Creed II.
- GTA 2 (PC). I didn't really care for GTA 2, at least not single player mode. I had a lot of fun playing some network multiplayer games with friends in high school computer classes but never got very far in the main game.
- skate. (Xbox 360). I'm not into skateboarding but the demo was pretty fun. I probably should've just stuck to that.
- Various Simpsons games, including Bart vs The Space Mutants, Bart vs The World, The Simpsons Hit & Run and The Simpsons Game (NES, PS2, Xbox 360). The NES games are brutally Nintendo Hard (to quote TV Tropes) and as much as I love them, the Angry Video Game Nerd was spot on with his critique about the controls. I've gotten up to the third level of Space Mutants and the third world of...World. From memory, I got up to the Apu stage on Hit & Run and I didn't get far into The Simpsons Game before I got kind of bored with it.
- Street Rods II (PC). This is one of my all-time favourite games but I was never able to win the Mulholland Drive race, thus I was never able to challenge The King and truly finish the game. I've played the game all the way through the three months of Summer but as I said, I haven't truly beaten the game. Still had lots of fun with it though.
- A Boy and his Blob (NES). Another Nintendo Hard game, for me at least. I've gotten into the underground caves, started collecting the treasure and...yeah. Could be completed with patience and a walkthrough to speed up the trickier parts.
- Castlevania III (NES). I'd say I've been able to get about halfway through the game, but the Castlevania games on the NES were notoriously hard for two reasons, as the AVGN has pointed out: there are some difficult jumps and when you get hit you fly back, often into a pit of instant death. Great game, but one I've always ended up putting aside.
- Kabuki Quantum Figher (NES). Yet another NES game where I've just run out of patience/become bored. It's actually a pretty good game though.
- The original Legend of Zelda and Zelda II: Link's Adventure (NES). I really should, though.
- Lands of Lore: The Throne of Chaos (PC). Good game, with a walkthrough I got a fair way through it but never through to the end.
That's about all I can think of at the moment but as with my other list, I'm sure others will come to mind.