Skyrim is obviously a graphical improvement. Skyrim's Dungeons are way better designed than Oblivion's were, and since you spend a great deal of time dungeon crawling, that's kinda of important. I prefer the Skyrim Main quest, cause i hated the oblivion gates, but at the same time i was way more invested in Martin's character than in Ulfric or Tulius But Oblivion's side Quests were way better, or at least the Thieve's guild and the Dark Brotherhood (probably my favorite quest in any RPG), although the Fighter's and the Mage's guild were boring as f$%k, Shivering Isles was awesome. There was more to do than simply go into a Dungeon or fetch this or that. The cities in Oblivion were actual cities, big, full of houses (even if you couldn't actually get into every door), full of people, they had more life. Skyrim is fully voice acted, and i quite like that. The new leveling system gives you a lot more freedom, wich in my opinion the more the better (more freedom in quests would have been great). Both were kinda imcomplete and only truly achieved greatness through mods.