Just the sheer scale of it! I find it quite fascinating that the world is SO big that it will take days of real time to reach a town in a different province if you walk. Sure, the terrain is all randomly generated bull in between the towns, but I still think that's pretty impressive.

It's actually sort of fitting that the spaces between the "important" parts of the game are randomly generated, as Tamriel was apparently the campaign setting for a D&D game that folks at Bethesda were playing. Dungeon masters make shit up all the time to make a world seem more fleshed out and real.

I really hope in the next generation they can start bringing some of that variety back in, I loved morrowind, oblivion and skyrim, but they do get more and more restrictive with each iteration. I know the older ones are even more expansive.