Neverwinter Nights: I was expecting another Baldur's Gate II, but the singleplayer campaign was a glorified version of Diablo and the engine was terrible (enormous rooms, empty mile-long corridors, every place looked the same).
Neverwinter Nights 2: I really wanted to play a good RPG again, and despite the overwhelmingly negative user reviews I went ahead and bought it. Ugh. I recently got a new GPU and NWN2 came with it, and I figured I'd give it another go, after the updates and all that, but the game's installer is broken. Ok, whatever.
There have been many games that didn't turn out to be as amazing as I thought they'd be, such as Half-Life 2, Mass Effect, Grand Theft Auto IV, BioShock... but NWN and NWN2 sucked so much I didn't even get halfway through them.