I've never played through the other TES games. I was too young in 94.. Also didn't have a computer. When Morrowind came out I tried it but I think I was still a bit too young to understand it all. So I'm a bit curious about what parts they have changed to appeal to a mass audience. To me the game is (almost) "perfect" but I don't have any other TES games to compare with.
There are too many examples to list concisely here, but I will give a fairly standard one. The number of skills, first introduced in TES II: Daggerfall, have decreased and simplified over time. For example, Skyrim has two skill sets for weapons: One-handed and two-handed. In Oblivion, those were divided into blade (covering all sword types) and blunt (covering all axes, maces and hammers). In Morrowind, those skills were divided further -- Short blades (daggers, shortswords, etc), Long blades (long swords, claymores, katanas) and blunt weapons (maces, axes, hammers). Prior to that, Daggerfall had axe as a separate skill to blunt, making the total number of melee weapons skills at four. This forces the player to specialize to a greater degree and makes leveling and character builds both more diverse and more complicated.