The fact that it's just SO EASY to fall to your death in some minds. And by some minds, I mean Boyd's, with its funny gravity and all that.
Also, Edgar's mind. with the bull and the hard-to-navigateness and THAT *%$#ING MATADOR. X_X Looked lovely, but that's about it.
I may not request that my brain is put into a Dalek shell instead of a brain tank.