Hmm, nice thread!
Top of my head, in no particular order
DM, the second : Hard
Wizardry, first : So damn long, i honestly got bored. Or so i remember, might be self projecting, ie got pwned somewhere and gave up, lol
Ultima 4 : Some bug that prevented me from completing it, i think
IWD : Got bored
IWD 2 : Got bored. Ok, i lied. I installed it, it launched, thought "fuck that shit", exited. Still think that. RTwP
UU the first : Got a nerdrage episode of severe proportions because i wanted to play it caster-only, which was nigh impossible due to the engine; could not aim at close quarters. Completed years later
Skyrim : Fuck that shit, literally a couple of days of modding and i still couldn't. I then saw the Crispy thread. Couldn't change my mind about Skyrim, but it sure did about Crispy
Oblivion : Fuck that shit, i thought i was getting Morrowind 2.0
Arcanum : Don't even remember why. Just another nerdrage it would be many years before getting over and trying it again
WL2 : Don't even get me started. Fuck them, fuck them all
PoE : Got bored
Beyond Divinity: Was willing to forgive so much, but not all that mindless repeated grinding inbetween 'zones'. Details escape me, some special instance you were 'encouraged' to visit over and over again in between the proper levels?
Gothic 3: It was not Gothic
Risen I,II and III: It was not Gothic
Witcher 2: If i wanted an interactive movie Telltale game, i'd have gotten me one. Except i don't. Effect slightly lessened by my occasionally being allowed to click a mouse button. Effect not sufficient.