Skyrim is not graphically impressive. It's in the average range in terms of open world games.
except that's [censored].
Assassin's Creed II looks pretty much terrible http://cloud.steampowered.com/ugc/542888921690793054/6B5BA3344E1F7F4A53724A7436063F5071C0C1DE/. RDR looks great from a distance, like console games are obviously supposed to be played, but look at it up close on a computer monitor and you can see every single cost-cutting trick in the book, from painfully low-res textures to [censored] shadow draw distance to Rockstar's signature horrible LOD streaming.
Skyrim looks better than Risen, Arcania and Two Worlds 2, and those are the only other games on the 360 that are both open-world (though the latter two aren't, really) and artistically comparable. Crysis 2 is not legitimately comparable because it is not open-world and does not have a multitude of complex underlying AI or physics systems (indeed, nothing else does).