I think it's mainly the heads. I know that regular minifig heads may not have looked great either, but all these special molds go well beyond the things I consider to be cornerstones of the LEGO style.
While I respect your opinion, I am a bit surprised. The two things which really sold me on this line are the shell "backpack" and the accuracy of the turtle's head molds compared to that of the show. To each his own, I guess.