I think it was the atmosphere, Kovacius. Oblivion and Fallout 3 just don't have that "Wow, this is a real world with real people and a real culture" feeling. Don't get me wrong, I still love OB and FO3, but they just don't have that magical, lifelike atmosphere.
I totally agree, once you had access to red mountain through ghost gate, getting access to the Acrane University just did not compare. Seeing the different landscapes and wondering where each tunnel would lead you in morrowind. and exploring the same landscapes and knowing what exactly was in each fort that lead nowhere in oblivion.
Oblivion also didnt have the lore that morrowind had. you HAD to learn the lore of the land to progress in morrowind, in oblivion all you gotta say is 'ok tell me the name of the guy i gotta kill and let me follow my pizza slice'.
We need atmosphere, lore, and landscape first and foremost imo.