Oblivion is part of an upward tend in some ways too. The combat, stealth and magic systems are all greatly improved over Morrowind.

And as you say the sidequest writing is strong. Although personally I prefer the less-complex but more numerous quests of Morrowind or Daggerfall, they make the game feel less linear and increase replay value. There's no point to a long, inctricate, linear, well-written quest if I've already seen it 5 times

But the sidequest writing quality in Oblivion was a high point of the game.