I think I may have answered this here before(which I say only because if people look in archives, I might not be saying exactly the same thing each time--but the gist is the same)
I think the big mistakes were that we ignored basic tenets of warfare (guard your flanks), put far too much trust in assumptions that had no basis (You'll be liberators "then why all the bullets?" there will be weapons of mass destruction and the world we see the justification for our actions)and we never asked "How will they try to win? (how can they win)" which, i argue, is a fundamentally different question than how will they fight
The manpower was certainly significant as was the type of manpower (reserves and guard--great people wonderful Americans but you shouldn't use your strategic reserve to start a war)
I don't think anything will change--Pace seems like a good man but they all got their stars in a peace time military