Bethesda can never make a thing better than Shivering Isles in my opinion.. I had a lot of attachment to that Expansion.. I don't know if it was because I never researched a thing on it before buying it or it was the DLC itself.. Unlike the base game of Oblivion, I felt like I was in a world unlike any I have ever witnessed.. As for Skyrim, I didn't really like the environment.. It may be that I live up north in real life so the environments are basically the same.. Bethesda still wrote fun quests at that time aswell, each very memorable.. Also they added in a very big city that was very dynamic..
They did? Where?

Seriously, first time I got to Windhelm I was like, "wait, is this that SI city?" Anyway, I didn't think so on any of your points as to what made SI "great". In fact, I feel quite the opposite, more like it was very repetitive tedium. And, well, Sheogorath was there, so another huge minus for me.