The one that came to mind first was Spore, since I practically lived and breathed that game until it came out. I watched every video three dozen times, I read every article and update, I bought the Creature Creator kit. I played it for hours when it first came out, and enjoyed it to an extent. Then I quickly realized how terribly shallow and inconsequential the game was. There was no flow from one bit to the next, and you never felt like the decisions you made in the early game really mattered all that much in the later game. There was really only one major decision, which was carnivore/herbivore. Everything past that was just an extension of that basic decision.
Next up would be Fable 2. I bought a 360 just for that game. The controls were abysmal, the gameplay was barely there, the characters were uninteresting and irritating, and I never really felt like my character was an extension of my play style. There should never have been a Fable after the first.
Elder Scrolls IV, Oblivion. I'd still rather play Morrowind. Remember those early videos of the emergent AI, and how it reacted to the things the player did. Yeah, that worked. The sense of exploration is all but killed by the fast-travel system. The combat was unintuitive and went from way too easy to way too hard, with barely any middle ground. Level scaling was a terrible idea, especially when it led to beggars carrying around daggers which were worth enough to buy a damn house. Then they blanded down the intricate and unusual setting of the Elder Scrolls world and made it into a very typical fantasy setting. The game did a few things right, but overall it was so disappointing.