I'm only gonna mention games that I've started on multiple times and/or WANTED to complete.
Pools of Darkness: I played through all 4 games in 2000, by the time I was hopping between dimensions I was burned out, only just managed to beat Thorne and then be done with it. No, I'm not gonna try again.
Syndicate - American Revolt: I beat the original, but the expansion pack is one of the most brutal games out there. I had 3 missions left by the time I packed it in. (To this day I only know of one guy who has completed American Revolt.)
Gothic 3: Too big, feels too much like an MMO, and of course all the bugs. Community Patch ain't gonna fix this.
Legend of Faerghail: I spent too much time on trying to beat this game. Bad translation from original German wasn't helping for some of the riddles, but then later I realized that I was playing an older, buggier build than was the standard, even though it was a retail copy.
Space Hulk (1994): While I got pretty far, I could never beat the later missions.
Albion: Mandatory grinding killed this game for me.
Icewind Dale - Trials of the Luremaster: I've beaten the original, I've beaten Heart of Winter, but never this. By that time I just don't feel like going on for some reason.
Lost Vikings: I reached the endboss, but gave up on trying to beat him.
Buck Rogers - Countdown to Doomsday: I played this much earlier than the Pool of Radiance series, and as a result I wasn't quite as "aware" of what needed to be done. I got as far as Mercury, but that was it.
Spellcasting 201: I could never beat the first hazing puzzle. When I consulted a walkthrough years later I understood why, you really can't waste any time exploring in this game.
Cave Story +: Twice now I've tried to finish this game. Twice now I've given up in frustration because some insignificant decision on my behalf ruined my chance of getting the true ending of the game. (Later I saw a video of the last part of the game, and I don't really feel bad about giving up.)
Descent 2: I beat D1 and D3, but D2 always kicked my ass, the L12 boss in particular. Sadly I think the Descent games have gotten too old for me to try again.
Lemmings: There was one level that completely stumped me back in the day about 2/3 of the way through.
Privateer: That escort mission out of Oxford proved too much for me.
Star Wars KOTOR: I had a go at it once. Once. [/Joe Piscopo]
Deadly Rooms of Death 4 - Gunthro and the Epic Blunder: Considering I've beaten the other three games in the series (and two of them with 100% completion rate) I find it a little troublesome that I can't bother completing this one. It's just uninteresting. It's also the reason why I haven't purchased DROD 5 yet.
Titan Quest: This was just earlier this year, I played through a boring Diablo clone until the endboss proved to be a Wall of Difficulty. I saw no reason to bother, Uninstall.exe.
Warhammer - Shadow of the Horned Rat: It's deliberately one of the hardest games ever made, finding the "right path" through it is a challenge in and of itself. I never even got to Athel Loren.
Warhammer - Dark Omen: Somehow not as fun as its prequel, never interested me that much.
Zeliard: I always thought I'd just scratched the surface of this game. I recently watched a LP and I realied I had reached past the half-way mark. Shame the game is shit.
Grimwulf said:
Actually, we need a "I'm surprised I ever finished it thread".
I agree, bet there would pop up a few surprises from that one.
CryptRat said:
I remember being stuck by text enigma at several games like Legend of Fearghail
Glad to know I was not the only one. The simple riddles were fine, it was that big-ass long one in the Dwarven Mines that was permanently ruined by lousy translation.