The whole game really.....Tolkein practically invented the modern fantasy genre. He's the first person who took elves beyond "Santa's helpers" and popularized the modern concept of dragons, trolls, orcs, etc.
While he did invent the concept, TES actually uses very little of what he created. Other than Elves (as the word, and the fact they have pointy ears,) and Orcs, TES really made up all it's own stuff... Sure the basis is the same, but TES goes FAR beyond any other fantasy game to differentiate itself, and honestly I think grouping it blatantly as 'entirely based on Tolkeins creation of fantasy' is somewhat insulting to all that work they did to make it unique...