Tons from the last generation. I didnt really do research on games before i bought them and so many of them sucked so terribly.

This gen, the only games I haven't completed are Lair, folklore, and this one shovelware game. On handhelds there are a bunch i haven't finished- i bought them for the gimmick controls or the radical storyline

The Last Remnant...the game was horrid...thank goodness for Gamespy!

psp:
wipeout pure - maybe i lost interest in those kind of racers, but thwe handling was so terrible that i was just hitting walls all the time,
Patapon - i still don't see appeal of this game and after a while it became just grinding for new items on the same 2-3 stages,
Hot Shots Golf: Open Tee 2 - simply terrible game and i though i would never play again fun golf game until i got Pangya,
Toca Race Driver 2 - i though that i would get a racing game not series of mini games on race track,
Monster Hunter Freedom 2 - too complicated controls made this game no fun at all,
Valkyrie Profile: Lenneth - at some point it just wasn't fun and i stuck,
gran turismo portable - after spending 8 hours in the game in 2 weeks i just sold this game, driving in this game felt like a chore and the fact that even on highest difficulty setting i was just driving by AI and never met them again was dissapointing. bad game even as a pick up & play title. plus the music was annoying as hell(5 laps on nuburgring or that huge ass oval track while hearing the exact same annoying track in a loop was just tiresome),

wii:
Marvel: Ultimate Alliance - i though it was great and then i played it, this game is doing pretty much 90% of things for you:/

Yes, on several occasions.. The more recent ones I are:

Red Steel - Novel concept at the time, but the controls were pretty horrid and the single player got boring

The Conduit - Got close to beating it, but just got bored in the single player, and the game tended to make me nauseous

Mass Effect - Not really bad as much as I just couldn't nail the controls and the battle mechanics. Just didn't click for me.

RE5 - Bored me, and seemed too much like RE4, just in a new environment. Didn't play this one much.

Manhunt 2 - Somewhat sluggish controls and boring gameplay after the novelty of the finishing moves wore off ultimately deterred me from finishing it.

Vampire rain
Beowolf
overlord raising hell

GTA4 and Dead Space Extraction, two horrible and over rated games that I couldnt be arsed with.

I submit my game collection and rest my case. I get bored easily and have poor judgment when craving new game.

Driver Parallel Lines - That game is aweful

Alone in the Dark - Another terrible game I bought.

Dynasty Warrior: Gundam 2. It's just pointless repetition over and over again.