I don't hate KOTOR, I just think it was ok, with some noticeable flaws. Just like Mass Effect.
Regarding level design, ME is almost the same as KOTOR with levels consisting of a peaceful zone and a combat zone attached to each other. The main difference in level design is that, given the nature of the combat, the level design in ME affects the way combat develops, while in KOTOR it really just provides ground to stand on while fighting.
As for art design, it's generic sci-fi stuff in ME and generic Star Wars in KOTOR so I really don't see what you're on about. I guess ME is better because it uses a snazzier engine which allows for more detailed textures and models. In KOTOR, I liked the Wookie planet with the huge tree trunks.