I think Oblivion's reception was mostly in relation to how people felt about Morrowind. Oblivion simplified a lot of what Morrowind did, which drew some ire. Atmospherically, though, people weren't happy that they moved from Morrowind's much more unique fantasy setting, full of massive fungi and swamp-striding bugs and weird demigods, and moved to a much more generic looking medieval countryside. That said, I also appreciate the game's reassessment, because I think some of the factions quest lines are among the best in the series.