NWN, I was expecting so much more (and I was shocked at the bad graphics). Second would be Temple of Elemental Evil. Again, this game could have been so much more. Both games I still enjoyed enough to finish (so I still liked them), but they were disappointing in the fact I was expecting something a lot better.
Third would be Morrowind. This game had such potential, but ended up just being repetitive. I loved it at first but after 40 hours in I was doing the same thing over and over again.
Warcraft 3 would be number 4, I expected something more innovative. But nevertheless I still finished the single player campaign, and enjoyed it.
Other games like Dungeon Siege I knew was going to suck, so they met expectations rather than I was disappointed.