These are just some of my categories and a few of my examples. What categories would you contribute and what games have you NOT finished for the already stated reasons?
Diffculty never for me. Unless its like an NES like game, I can get through basically any game.
For me most of the time its distractions, something better comes out. Or I go on a long trip. Too much content happens as well, its why I stopped playing Skyrim for months, I stopped playing Xenoblade, etc.
I was also thinking of a category like Motivation or Incentive or Pacing, but I don't know if this would be the same thing as Boredom. Sometimes you'll enjoy a game well enough, but it's missing that ONE thing you just can't place. You can't give an exact reason why you stop playing something, but you do. I think you guys were discussing this somewhat on the podcast where you were talking about Game Design or the lack thereof... The developers failed to drop those subtle hints or introduce content with enough consistency where you just lost interest... Know what I mean?
Losing a game save is almost an instant killer for me. Happened to me in FFXII, never finished it, dont want to do it all over agin.
Depending on time, I might go back over the last year or two and try to put the games I bought and DID NOT finish into these categories, just for the heck of it... We'll see if anyone shows any interest in the topic first!
**I bought LOTS of games... 
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Resonance Of Fate, I like the game a lot but good god is it a grind-fest. The difficulty level is absolutely insane (It's even harder than Dark Souls IMO).
Difficulty -- portal 2. Hard Corps: Uprising. Catherine. BloodRayne: Betrayal. Dark Souls. Eufloria. Professor Layton Unwound Future / Last Spector. Pushmo. VVVVVV. Closure.
Distraction -- Dragon Quest VI: Realms of Revelation. Trials HD. Adventures of Shuggy. Boulder Dash XL. Half Minute Hero. Trenched. Bit. Trip Saga. Kirby Mass Attack. Rochard. Aliens: Infestation. Rayman: Origins. Saints Row the Third. Mario Kart 7. Mighty Switch Force. Mutant Mudds. RE: Revelations. Rhythm Heaven Fever. Asura's Wrath. SSX. Devil Survivor 2. Dillon's Rolling Western. X-Scape. Tales of Graces f. Anamoly: Warzone Earth. Rainbow Moon. Skullgirls. Shoot Many Robots. Defense Grid.
Boredom -- Viva Pinata: Trouble in Paradise. Phantasy Star Universe. Super Scribblenauts. Okamiden. Crysis 2. Pilotwings Resort. LA Noire. Red Faction: Armageddon. The Legend of Zelda: Ocarina of Time 3D. Resistance: Fall of Man. EDF: Insect Armageddon. Lara Croft and the Guardian of Light. DeathSpank: Thongs of Virtue. Resistance 3. Crimson Alliance. StarFox 64 3D. Sideway: New York. Lord of the Rings: War in the North. Sonic Generations. The Legend of Zelda: Skyward Sword. King of Fighters XIII. Fortune Street. Kingdoms of Amalur. Lollipop Chainsaw. Heroes of Ruin. ICO & Shadow of the Colossus Collection.
Bugs -- Two Worlds II. Ghost Recon: Shadow Wars. Dead Island. BattleField 3. Skyrim (PS3). Silent Hill HD Collection. Prototype 2. FEZ.
Content Overload -- Shin Megami Tensei: Overclocked. Disgaea 4. Rune Factory: Tides of Destiny. XenoBlade Chronicles.
WTF? -- Banjo-Kazooie: Nuts and Bolts. WarioWare D.I.Y. Brink. Dream Trigger 3D. Resident Evil: Mercenaries 3D. SolatoRobo: Red the Hunter. Nano Assault.
Undefined -- Nier. LittleBigPlanet 2. meh! Dead Space 2. Bulletstorm. Killzone 3. Dragon Age II. Yakuza 4. Shadows of the Damned. The DishWasher: Vampire Smile. Insanely Twisted Shadow Planet. El Shaddai. Deus Ex: Human Revolution. Gears of War 3. Dungeon Defenders. Uncharted 3. COD: MW3. Shinobi 3D. Theatrhythm Final Fantasy. Rhythm Thief and the Emperor's Treasure.
So, it looks like I either get Bored or Distracted, for the most part; a few more Undefined than I thought!
In many cases I just get burnt out on the game. I just get overly excited and start doing everything I can to prolong the experience, finding every collectable, doing every side-quest, anything I can for hours and hours. Eventually I lose focus of the actual game, and I still don't want to beat it because it'll end, but I don't want to do more optional stuff because it's boring. I tell myself that I'll give it a few weeks and maybe I'll want to continue, but by then another game has caught my interest, or I've forgot what I was doing in the original game and may as well start a new file at that point.
Listen to Iced Earth and play Doom
You've done a good job of summarizing the reasons. It has only happened to me a hand-full of times over the last few years.
Usually it's when a game turns out to be way bigger than I thought going into it (Okami, LKS, Chain of Memories). Where I am about 25 hours in and then I realize I'm less than half-way done.
Very occasionally it's difficulty, that's happened twice with Darksiders (where I went away from it for a week and by the time I came back I had lost the controls) and DKC2 where there was one of those crazy bee levels.
I try to beat most games within a week as I've found dragging it out beyond that can make a game feel like a bit disjointed and boring.
Difficulty combined with boredom is the worst.
Mostly boredom. Difficulty can also kill a game for me, if it becomes too frustrating and/or annoying.
Dvader said:Losing a game save is almost an instant killer for me. Happened to me in FFXII, never finished it, dont want to do it all over agin.
This has happened to me in Metroid Prime 2. I was nearing the end of the game, and my memory card became corrupted. I lost all of my GC saves. 
aspro said:Psychonauts killed my 360.
How?
Now I think about it it is amazing I'm on my fourth 360.
aspro said:Psychonauts killed my 360. I'll never go back and replay it.
Like Foolz said,difficulty combined with boredom is a one-two punch that I can't get around. Difficulty on it's own usually won't do it. Off the top of my head I can only think of two games that were genuinally too hard for me to keep playing and I had to go in for the mercy kill and play something different: Monster Hunter Tri and Dark Souls.
Just wanted to start a little discussion that some of you will be able to relate to, others won't. Other than the game itself actually sucking, why do you most often --NOT-- finish a game?
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Difficulty -- Nothing's worse than playing a game, getting pretty far into it, then running into a wall. You'll try, try and try again only to fail. Say it's an action game and not something you could just FAQ to find a solution to. Try as you might, you just cannot beat something to the point where you say "FUCK IT! THAT'S IT!" and give up.
Distraction -- Months ago, I bought Shin Megami Tensei: Devil Survivor 2 for the DS, started playing it... Then Pokemon Conquest came out, and I thought I'd give it a try. It was sooo unique and well done, I thought to myself: "Eh. I can always go back to Devil Survivor." As I played Conquest, I realized "I don't know half of these Pokemon! Maybe I should play a little of Black or White..." Month or two, and a hundred Unova Pokemon later... still playing Pokemon Black... and New Super Mario Bros 2 comes out tomorrow! When I am at home and want to play a console game, I have about 60 DLC games, alongside a handful of Disc based games, the "maybe I'll try this, maybe I'll try this" is even worse at home!
Boredom -- Kingdom of Amalur put a fresh, colorful, unique face on the open-world, RPG genre. New Mythology, a dream team of developer, artists and writers... it had nothing but promise... at first. I played it A LOT got pretty powerful, pretty quickly then noticed I wasn't having a hard time beating the enemies. Switched it to Hard and played on... Button mashing and the utter lack of challenge made me take the game out of the PS3 and that was pretty much it... There's also quite a few games suffering from the "Been there, done that Syndrome." Some games are sooo plain, so vanilla, or bring absolutely NOTHING new to the table; Heroes of Ruin is the text-book definition of THAT!
BUGS! -- (Not Bugsie or Bunny!) Falling through floors, getting stuck in walls, scripted events not loading, quest markers/characters gone missing. Hard locks, corrupted data... Since the dawn of online-enabled, patchable console games, developers have been empowered to put out unfinished or insufficiently quality tested "Finished" products. Fallout 3 fell to this type of flaw. Oblivion was cursed to oblivion for the same reason. I'm not as picky as you would think though, as Mass Effect 3, Skyrim, Metroid Prime and a handful of other games had their share of problems too, but I persisted!
Sooo. Much. Content! -- Ever stop playing a game because there was TOO much to it? You just get sooo overwhelmed, you are like: "Well, I'm done!" I'm looking at YOU XenoBlade Chronicles!
W.T.F.? -- Try as you might, you can't figure out what the FUCK the developers were thinking when they made THAT game! In the attempt to make something unique, they passed into the realm of such utter obscurity, you just couldn't figure the damn thing out. Try playing Resonance of Fate for a PERFECT example of this category!
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