Well:
Number 1 biggest offense:
This just killed Skyrim for me. So so so much gameplay potential and fun missing. Fuck, half the game in Oblivion was coming up with dumb weird spells for your build, either a paralyzing touch so you could beat their lifeless ragdoll body, or stacking a bunch of slightly different named speed spells so you could run so fast you broke the loading of the cells. Or how about a fireball the size of a city block that moves slower than shit? So much fun and freedom, completely gone.
Enchanting also got just about as equally neutered.
Also, in general, Oblivion had:
-A better, happier, less depressing to be in world. Why do I want to spend 500 hours being gloomy and depressed? Skyrim has its pretty moments, but most of the map is glum city.
-Better music. Skyrim has some amazing music but a lot of it is annoying and depressing and I had to turn it off 20 hours in. Also, the best songs in skyrim are rare. None of the music in Oblivion actively annoyed me.
-Better quest themes and quest design. This has been said to death in here already. Not every quest sends you ALL THE WAY ACROSS THE ENTIRE MAP.
-More interesting lore and exploration of world lore.
-Better houses to buy, and you could decorate them with physics objects from your inventory. This was completely broken in skyrim and after reloading the game all the objects would be misplaced or fallen through the world.
-One of the last "expansion packs" in gaming ever, and it was amazing (Shivering Isles).
-Guilds were way deeper and the arena was awesome.
-I like the enemy variety and opposition better in Oblivion personally.
-I actually enjoyed the Oblivion realm. I thought it was fine and added more to the game than just caves and ruins on the overworld.
-Both games are broken in terms of scaling and difficulty, but I found Oblivion a little less so, but maybe that's because I was more inclined to min-max everything in that one.