i would. The graphics are definitely better than oblivion. the most obvious is the character models, but it is easily noticeable in the environment as well.
google image search oblivion vs skyrim, the differences are glaringly obvious. i don't see how you can imply that they are not improved. I think your comment about the only difference being tree types is pretty ridiculous. example(morrowind, oblivion, skyrim):
http://www.tesnexus.com/imageshare/images/128909-1303151790.jpg
http://www.tesnexus.com/imageshare/images/128909-1303151790.jpg
it seems to me each title is meant to convey a certain feeling or attitude. although the game is by definition open, the stories are crafted around specific races/character types for each game.
morrowind was dark magic, oblivion was righteous religious warrior, and skyrim is nordic badass. even if your character doesn't match the theme of the game, it is very definitely a theme.
as for art style. I really hated how smooth everything was in oblivion, skyrim has texture and dirt, it's much more immersive imo. so while i can't say the graphics blow you away compared to oblivion, they are very definitely better. I have already had more awe moments with the scenes it creates than i ever did in oblivion. as someone else said, this is the sequel morrowind deserved