My list:
1.
Oblivion. I loved Morrowind, but Oblivion took out all the things that made Morrowind fun and replaced them with things I didn't care for, like levelled list, tortured corpses and the worst sort of hell: the same Oblivion instance over, and over, and over again.
2.
KoTOR2 Again, I loved KOTOR1, the twist got me by suprise. KOTOR2 just didn't do it for me, the story didn't suck me in, I couldn't care less about the NPCs, I just wanted my Carth back!
3.
Dungeon Siege2 I liked DS1, unlike most of you. I appreciate simple hack'n'slash at times, but then it should stay simple. DS1 had that right, just like the original Diablo. DS2 tried to stick all sorts of nonsense onto the game, and had a horrible Save and Death system, taking away from the simple click-click, kill-kill that made the first one enjoyable for me.
4. Divine Divinity 2. Being tied to your nemensis, a dmon from the worst pits of hell, would have been a lot more enjoyable if he didn't sound like Fozzie Bear from the muppets. DD1 was fun, even though it did slack the pace at times and lacked direction, but DD2 was bad.
5. Final Fantasy X. We bought a PS2 earlier this year, and I couldn't wait to try the Holy Grail of JRPG... only to find it was really a movie where I occasionally got to walk three steps, and if one of those steps was in the wrong direction I got punished with tedious turnbased combat that took way too long. If there would have been any way to skip that I would have grabbed a bucket of popcorn and appreciated the movie. But after being forced to play some sort of underwater rugby (WTF? I hate sportsgames, so what is something like that doing in my RPG?) I gave up.
There have been other games that just weren't to my liking (like BG2, and the Fallout series,to give examples) but that was just me. i bought them to try them out and didn't like them, no biggie. These games however I bought after either playing the game in the series before them, or because I bought the hype, and so I class them as dissapointing.