Fable - Because it's Fable. It wasn't that BAD, it just needed a lot more content and more of a purpose behind the whole good/evil thing to justify the price tag and permanent ownership.
Final Fantasy XII - Worst RPG ever. Worse than Shadow Madness. Within a few hours of "playing" it I was asking myself what the main character was doing there and why he hadn't left yet. Within a couple more hours I realized I wasn't going to receive an answer and this game was even more rushed and pieced together than FFVIII, which I wasn't stupid enough to buy.
Xenosaga Episode Anything - Thank God I got Episode I used out of a bargain bin many, many years after the fact. I'd been desperately in search of something creative. This offered something of the sort, but without the common decency of offering a GAME inside the package.
Grand Theft Auto: San Andreas - The game I never would have bought if I knew what it was actually like. I loved being an Italian mobster in the previous games. I HATED being a gang banger in this one. I hated the environment, I hated the characters, I hated everything about it except the gameplay. I played it for ten minutes, got to the point where a cutscene started wasting my time with one of the protagonists threatening to beat his wife, discovered the pizza joint was a bunch of puns on breasts, and realized that everything about the main character and everything in his home were allusions to having a large penis before putting this game down and never picking it up again.
Grand Theft Auto: Vice City - Special case. I bought a used copy at EB before they were acquired by GameStop. You couldn't return used games for a refund if you'd actually opened them, even if something was wrong. Well, in this case, something WAS wrong. My disc was scratched and I couldn't get through more than a few minutes of gameplay before the PS2 stopped reading it. It was a pity because I'd played this game many times before and was DYING to finally get my hands on my own copy. This one was only $6 at least. You get what you pay for, I guess.
Phantasy Star Universe - It's not like I thought Phantasy Star Online was that great, but in all honesty it's a much better designed game than its successor. On the exterior PSU is more polished and more visual, but the environments don't have as much thought in them, they repeat ENTIRE MAPS repeatedly and recycle levels in a much more linear fashion, and since the special attack attribute has been separated from weapons themselves that gives weapons little license to have variety, the variety of its unique weapons being the only thing that motivated me to play 50 hours worth of PSO in the first place. Overall not worth what I paid for it, sold back in a month.
Dead or Alive 4 - All the good costumes are gone and more than ever it feels like winning depends on luck. I'd played DOA2 Ultimate and loved the series for it, but something about 4 felt like a SHARP step backwards in controls. Felt like I'd explored the whole game after a few runs through arcade mode. Didn't feel the need to play it any further. Hated multiplayer since it felt like luck was dictating the game more than skill. Sold it back within a week.
Armored Core 4 - BECAUSE IT'S ARMORED CORE. This was supposed to have been a big leap forward, but it somehow managed to be LESS FUN than previous games of the series. There just wasn't a comfortable way to control it.
You know, I regret buying 90% of the games I've bought in the past few years...