Revealing mistake: Just before the end credits, as Superman is flying past Metropolis and into space, watch the car headlights in the background. The background shot is blatantly sped-up by a large amount, as all of the cars appear to be travelling at huge speeds, even in a traffic jam.
Revealing mistake: During the airplane mishap where Lois is pinned to the ceiling, notice how the force is strong enough to hold her there, but both her hair and ID badge still hang down. (00:38:25)
Revealing mistake: At the baseball field, when Clark is holding the plane's nose he is replaced by an obvious CGI figure with zero facial expression.
Answer: There are two stories: a) the suit was made by Martha Kent from the blankets which came with Superman when he was a child and these blankets got superstrenght from the sun too. b) Superman generates an aura which protects him and his suit against such forces. Both stories can be true: either the suit itself is weakend by the kryptonite or Superman's aura was weakend by it. Currently it is said that the blanket-story is the correct one. Also, they don't actually cut it off him - it appears to open along a predefined line (coupled with someone exclaiming "got it!"), presumably the method Superman uses to get into the suit himself.
Bjoern_Buller