The trouble is, if you're some kid dumped at a VCP in the middle of the desert with other kids and a couple of creepy older guys, when IS roll up firing on full auto you don't know whether you're facing a crack unit of captogen fuelled Chechen renegades who are in competition for how many people they can behead with 1 hand or some buffoons who are shooting each other in the back and some guy who has just burned his mate's face off with RPG backblast.
It can't be explained as an inherent defect of character or lack of martial spirit in "the Arabs" since the Shia militias were subsequently able to pull the fat out of the fire. The correct raw material was evidently there in society.
There was obviously something seriously wrong with the "new" Iraqi army at that point, but IS must also have had some people who knew what they were doing to be able to get things to that point. It may indeed be fun to laugh at their ineptitude when the opportunity presents itself, but while their state may be crumbling now we should never underestimate their capability. They have the potential to be a serious threat to stability in the region for many years to come.
This is a lesson that we really need to take to heart, as apparently we didn't learn from Libya. We may be able to tear a state and society down, or at least help in tearing it down, but our ability to rebuild useful state structures and less formal social ties is far less than our ability to destroy them.