One major problem is that the error messages need a lot of work. Why aren't class variables and static methods not accepted? I can't know that if your code just throws an exception while iterating some dictionary.
I agree that error handling is one of the main things to be improved. The problem is that in some cases the AST walker ends up in unhandled states and prometeo throws a generic exception with a line number only. Are you looking at something in particular? With basically 0 users at the moment, this kind of feedback is quite useful.