Demon Souls. Don't worry.... I blame myself, not the game. I just didn't have what it takes. I was able to beat all the -1 bosses and the Tower Knight, but that FlameLurker really got to me bad. almost 2 weeks wasted, then I had enough.
Jak and Daxter Collection. I love Ratchet and Clank. So i'm like "let me try J&D" =D. The first one was alot of fun, despite being way too forgiving (This was at the opposite spectrum of Difficulty. Waaay to forgiving with deaths) But Jak 2 was atrocious. The missions had way too many sections reliant on sheer luck. Specifically the city missions. Skill could only take you so far in that game.
Fable 2. This was the purchase that made me stop watching X-play forever back in 08. I didn't have a PS3 back then, so MGS4 was out of the question. I had to decide between Fallout 3 (Which I have spent well over 400-500 hours playing these past 4 years) and Fable 2. Xplay gave Fable 2 GOTY and like an idiot I bought it. It didn't even work for a whole week until I learned I had to install it due to a bug. then I realized how boring it was, with it's fairy tale like story. (yeah I know...should have done my homework...) horrible RPG imo.
Although I'm sure I've got a laundry list of these, two titles come immediately to mind:
Cross Edge. I tried so hard to make myself give a crap about anything in this game and I just couldn't do it.
Final Fantasy 13-2. Truth be told, I feel like one of the few people that didn't think the original was "terrible". Not phenomenal or anything, just not bad. But the sequel actually made me kinda like 13-1 less by extension.
If I regret buying something then I feel I have wasted money, so I will enjoy a game enough to get the money's worth.
In other words, I have never bought a game that I put down the control and said, "I will never play this f**king game again" before I finished it. I may not platinum games I don't LOVE, but I will beat any game I buy.