1. Pool of Radiance: The Ruins of Myth Drannor. I loved the old gold box SSI games, so I was hyped for this one. Started of okay, but then got into the first dungeon and it was 3 or 4 large levels that all looked exactly the same, and it was a key hunt through all the levels, being attacked constantly by the same enemies. Over and over and over again... Then I think I got bit by a bug that made one of my critical keys disappear, or never appear in the chest it was supposed to be in or something like that. Never played it any further.
2. Ultima 6 and 7. Absolutely loved Ultima 3, 4, and 5. Once they made the shift to the 3/4 isometric view, it lost something. The view was zoomed-in more, so you couldn't see more than about 30 feet around you which made it feel claustrophobic. They got rid of the amazing dungeon room puzzles that 4 and 5 had. 7 was particularly bad for inventory management. You could manipulate almost any item you could see in the game, and there were LOTS of them. It became just a mess trying to drag things around, especially if an item was underneath one or more other items. Combat was awkward and clunky, all of the tactics from 4 and 5 were just thrown in the garbage, you just watched your characters autohack at enemies like some RTS. Storyline was the only redeeming factors in these two games.
3. Jade Empire. Combat system completely ruined the game for me. It boiled down to rock/paper/scissors and using your sword required chi energy. Ridiculous. Disappointment was partly based on how much everyone raved about the game, and partly because other than the combat system, it was great.
Oh, and for the record, Oblivion and Fallout 3, while not without their problems, are excellent games.