But Oblivion actually got most things right compared to Morrowind. Game engine, NPC schedules, graphics, fighting system, magic system, stealth system, mod kit, and a lot of other things are all markedly improved over Morrowind.
But Oblivion got the most important things not just wrong but HORRIBLY wrong. Mainly level scaling, lore/dialogue, and immersion features (e.g. icons showing the location of everything on your compass that you can't turn off).
If Oblivion had made the improvements it did without sacrificing Morrowind's quality in other areas it would probably be the best game of all time. As it is I'd give Oblivion a C-