Oblivion. Such a let down after Morrowind
They completely butchered the experience/levelling system and dumbed the entire game down to satisfy the more casual console gamers. They also removed everything from the game world that made Morrowind so awe-inspiring and beautiful; it just became a dull standard medieval fantasy setting with nothing to set it apart.
Another thing that was a real disappointment was that feeling that you always had that you were going to stumble across something really special, but it just never delivered
Invisible Wars was also an enormous let down. Only ever played the demo, but that was enough for me to realise what the rest would be like.