I recently had to extend the Form_validation class and one thing struck me as particularly inconvenient.  It seems that a lot could be gained by taking lines 649 through 682 (the lines which address what happens when a rule returns FALSE) from _execute and placing them in a separate method.  This way, if I needed to override the behavior of execute or create another method which has similar behavior, I would simply be able to call a method once my custom validation function fails.