Ditto. It's a really good game, but unlike Morrowind the main storyline doesn't really do it for me. I've probably logged at least 100 hours though.

Double ditto, I love the game to death but man am I apathetic about saving the world.

The foremost title I can think that fits this bill for me would be Fallout: Tactics. I bought it a couple days after it came out, and now, all these years later, I've never finished it. Probably never will. Every once in a great while, I install it again, play through the first 2 missions and then I erase it because its just so.....boring <shrug>
Then there is always the matter of the original Doom, but as soon as I finish ShadowGrounds and Safecracker, Doom is going to be my next project.
On the non PC Gaming side, the worst case would probably be Breakdown. I was literally on the fight with Solus where you have to knock his ass down like 5 times in a row without a break, I'm guessing it was the last fight of the game. That fight pissed me off so much, I tried it SO many times and eventually I just gave up, erased the save and sold the game, so thats one that won't ever get finished.

Might be the mission where you have to destroy a stasis cell? That one has three protoss armies facing you, where you start in the inside of some ruins at the top of the map.
It's one of the hardest missions in the original campaign. The trick is to build a fleet of carriers, and a diversion army of fast ground-based troops like zealots. Send both to the left-hand side of the map and scout out their base. Send in the diversion, then use the carrier fleet to zerg the objective. Mission completed.

Dwarf Fortress... because there is no end, only a series of lose conditions
Ghosts and Goblins on Nes , brutally hard
Etrian Odyssey for DS , not hard, just very, very long

Battletoads on the Gameboy. One of the few games I just wasn't able to beat (and probably never will).

yeah. then you pass the city level and then the technodrome rapes your face.

Might and Magic book 1...the Dragon City Council.....Kicked my ass every time.

Another one I forgot to add: Xenon 2: Megablast. Tried to finish it many many times. Every successive play I got a wee bit further, memorizing the behavior of the enemies, until I was fed up with it and decided to ignore it for a couple of days. Only, if you left it long enough, you couldn't recall all the movements of the monsters anymore, basically losing all of the game play experience you gained before.
Now I think about it, that was a key part of the gameplay: memorizing the routines of the enemies. And I somehow enjoyed that...