I think it was the level scaling combined with the lack of variation, coupled with the fast travel. You never actually had time for an "adventure" because you spent most of your time in a loading screen just clicking your way through the game world. The big, far-reaching quests weren't there either. There was never one that really pushed you to your limits and tested your resolve. It was just sort of meh.
I just hated how it felt like I fast-traveled myself from place to place because the scenery was too boring for me to bother with. Morrowind was marvelous. Oh, and the lack of actual open cities. Meh. I hate, HATE loading into cities. I love free, open worlds.