Prog isn't really a genre as much as an approach to music. It is about experimentation, about creative freedom, about new ways of looking at and combining music, about always changing and pursuing something new. There are certain elements that often characterize prog, but as soon as bands try and run to a formula of what prog is, they have missed the whole point. To me, trying to define what prog is runs counter to what prog is.
...or something like that; i dunno, i just listen to the stuff.