I played Oblivion first, best game I have ever played, Morrowind was meh in my opinion, it was really the combat that made me mad
I would add terrain variety was lacking in oblivion, Level scaling was terribly implemented, It also lacked epic items, the fact that bandits had deadric weapons/armor made NO SENSE at all, the fact i could find "modern" books in ayeleid ruins deeeep in said ruins or even septims made no sense at all, and the all might gaurds who are all knowing.
Now that up there ^^^^^^^^^^^^^^^^^^^is things that are wrong WITH THE CORE GAMEPLAY of oblivion now my opinions
Insta fast travel takes one out of immersion fast travel between cities only helped but MADE NO SENSE since i did not experience my character paying for transportation services and if i were to walk only the game would become unenjoyable. It was lacking rare items in morrowind you had rare armor types: daedric, ebony, glass and so on then you had legendary things which were hard to find and when you found them you were like o hellz yes the best part is the majority of them did not involve quests in morrowind but you had to go off the beaten trail. Exploration...the compass of all knowing ruined this feature and made NO SENSE because it was never explained anywhere in the lore HOW MY character detects all of these locations WERE IS THE RPG IN THIS WTF!