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All of the above, unfortunately. It seems these days a game's got to be outstanding AND short for me to finish. :(

I think it's funny that "OP forgot to include something" is currently ahead. Apparently there are lots of reasons to stop playing a game. =)

Gundam Side Story 0079: Rise from the Ashes. That final encounter kicked my ass every time no matter how I tried to beat it. Having everyone gang up on one of the two bosses never even came close to working so I always ended up splitting my forces somehow. That would buy me enough time to take out one of the bosses before the other one finished off my team and came looking for me. The problem was generally that I was too damaged to win by that point.

Metal Gear Rising: Revengeance. I could never get the hang of the controls. I nearly beat it, but I just gave up and watched the ending on youtube.

I've never finished a Final Fantasy game with the exception of Theatrhythm and that's because it's a rhythm game. (Still haven't unlocked everything though). Almost at the end of FF8, just stopped playing it. No reason why. For FFX I lost my save data. I was pretty far in and at a good level and the next time I went to load it up . . .poof - no data. Fuck that game.

I've also never finished a Harvest Moon game despite owning almost all of them. There comes a point where you end up just stopping and you never pick it up again.

In fact, I haven't finished a lot of games. I have a bunch sitting on my shelf half or not played at all. /shame faced

No option for "minimalstic" indie games or walking simulators with "oh-so-emotional" stories?

Doom III. I think I got half-way (or less) through the game but then had to format my computer and I forget to backup my save files. Not being able to use my flashlight and guns at the same thing also discouraged me from playing. Too dark 4 me!

can i say battletoads... i could never finish it, not even using save states with an emulator.

Planescape Tornment sadly. Got to the last section of the game, temple or something where you go alone and... i had a mage character which couldn't even get through the first room of it...

FO1, FO2, Gothic, TES3...
I wish I had my PC when there were released, now I can't force myself to play it after I've played much newer games.
But one day, one day...

Witcher.... I tried a few times, few years within. And always end up going through the sewers entering the city and then bam, I'm bored...

Freespace 1, the mission before the final mission is a fu***** escort level. You have to defend your own battlecruiser against an enemy battlecruiser (while flying around in a fighter). The enemy is damn hard to kill, and your own is too dumb to move away. I don't remember how many times I tried to beat this mission, on my final try I managed to kill the enemy cruiser, and... my own cruiser had something like 2% health left... and I was so happy that I finally made it... and in a few seconds I saw how my own cruiser got destroyed by the shockwave from the explosion of the enemy, mission failed.

I never got back to the game again. I hate escort missions with a vengeance till this day.

Oftentimes, it's just because a game is too damn long, with a large emphasis in that reason being the large quantity of sidequests. The Elder Scrolls and Fallout are a good example with this (although at least I've beaten Fallout 3's main campaign), as well as Final Fantasy. I mean, Mass Effect may be a damn long game (and series), but at least it was linear to an extent. But all of them? Hell nah. I get so distracted that I eventually lose interest or forget about the plot entirely and feel I need to pick it up from the beginning to understand why I need to save this dude, or kill that thing.

For honourable mentions, I also stopped Golden Sun: The Lost Age because of that unbelievably dense Lemuria/Triton/whatever puzzle thrown in the middle. That was just...wow. I picked it back up a few years later and got past that part (although still haven't beaten the game).
Another similar issue was with Star Ocean: Till The End Of Time. No bad issues with the game's UI or gameplay or story (all of it is FANTASTIC, a beautiful game all should play, if you haven't), but I got to one boss that I just find impossible to kill.

Dragon Age: Origins/The Witcher: way too much dialogue.
Morrowind: Combat just doesn't work for me. I'm waiting until Skywind is released to get the story.

Only game I couldn't finish was Bad Rats ;p

Seriously though only game I wasn't able to finish but wanted to was Tales of Vesperia on Xbox 360 but finished 1st run and to unlock everything you need to play muiltple times but learned PS3 got the same game later but they used money they made from 360 version to remake it with new cities, new character, new dungeons, and much more so I would never get to enjoy the game completely but theres hope its coming to PC since two of the series are coming Symphonia and Zesteria.

Metro 2033

Go! Go! Nippon. I activated it only for cards, 'cause I don't like VNs. But when I farm for cards, I actually always play games for an hour or so in hopes of hidden gem. But I got bored of this one in a matter of minutes D: still continued this charade for some time in hopes that my opinion changes, but it didn't
The only entertaining thing was the fact that I gave my real name (I don't know why I did something so stupid) and it was funny when characters refered to me as he. So for a while I amused myself pretending that I'm trans man :D But then it started to annoy me so I started anew with some generic male name. It didn't help at all. This game is actually only game I've encountered where I've ever been bothered by player character gender. Normally, I don't give a fuck about that, but in this one playing as male irked me a lot. O.o

Boshy. ^^

Lego Harry Potter Years 1-4.

With no special actions on my part, the game deleted my save file 2 different times I tried playing though it. Haven't gone back since.

Ultima 2: Grind. So much grind.
Betrayal at Krondor: I did not much care for the combat system. Great story though, but too much of the boring combat.
Ultima 6: I really don't like this engine. Controls are awkward, and night is too dark. Stacks are also a pain. (Same holds true for the two Worlds of Ultima that use the same engine).
Daggerfall The game ate my saves.
Valkyria Chronices The writing is cringeworthy. I might try to play it again and just ignore the interaction between the male lead and the girl you meet early on, because the game itself seemed to be pretty good.

Owning Bioshock 1 and 2, but i could not stand the gameplay, somehow.
I can't tell you what, but something was annoying me, so i quit.

Fallout new vegas hardcore mode.

Ratchet & Clank: Going Commando. I really wanted to like the game because its mechanics seemed fun, but i felt that its level design is total bullshit. About halfway through the game, it starts spamming the living hell out of tough enemies, and it just refuses to give you a break. And if you die before the checkpoint that's still a ways off, you have to deal with that shit all over again.

My philosophy is, that i'd rather have a comfortable, enjoyable experience than a challenging one. Ratchet & Clank doesn't seem to give me that leisure, although i'm still gonna give the third game a shot since that's appearantly the best one.

Half-Life 2

First I was annoyed by not getting the achievement for picking up the crow bar, but when I didn't get the " Keep Off the Sand! " achievement for countless tries I stopped playing.
I'll eventually go back to it, but not just yet.