As we said at the beginning, this is more than just an expansion pack. So long is it, and so radically different from the world contained within the original disk, that Shivering Isles may as well be called a sequel. At the very least, it’s a refinement of Oblivion - with a smaller area to play with, Bethesda has been able to really work on the world they’ve created, and the end result is something which just feels tighter, more focused and just plain better than before.
Take the NPCs, for example. While you couldn’t say they were more “well-rounded” exactly - in fact, far from it, they’re insane - Bethesda admits that their Radiant AI system wasn’t used to its full potential in the main Oblivion quests; this time they’ve been given a blank check for mayhem, with the smaller world and way-out setting enabling them to create some truly unforgettable characters.