Newt, Paul, Santorum, and Mitt can all speak for 5 straight minutes on any topic, sharing their ideas and thoughts and suggestions and criticisms.
Rick Perry and Cain are something different... They're working hard to recite the memorized talking points which have been written by their campaigns. If you create the ideas on your own, it's EASY to remember your 3rd agency you'd remove or how you would have done Libya differently.
But Perry and Cain are just cheerleaders for a memorized script. It works when you're just giving fiery speeches, but when you're asked tough questions, you dont have that knowledge and experience base to draw from.
Cain knows he is supposed to criticize obama on libya, and that one-liner works great in speeches.
But when pressed as to what he would have done differently... he has nothing.