There's never been anything groundbreaking about it, it managed to be just the right amount of dumbed down for console audiences while also solidifying the core "quest>dungeon>loot>sell loot >quest" loop that turns it into the addictive hole that people fell into when they played it for the first time.

edit: also helps that it's not fundamentally busted and ugly like Oblivion (which really doesn't even have much to offer over Skyrim besides its goofy and inept charm)