When the U.S. constitution was written up slavery was legal and women did not have the right to vote. On those two counts the Iraqi constitution is already ahead of the game. Personally I'm not at all worried about Islamic law (ala the Taliban or the Ayatallahs) being written into the constitution - no way that is going to happen in my opinion. If their constitution guarantees basic human rights, fair trials, the right to vote for all citizens, it will be far ahead of what pretty much any other ME country (other than Israel) has in spite of imperfections.
We seem to foget that around here...
If you will remember recent history, the former Soviet Union also had its version of basic human rights, their version of fair trials and an extensive slate of candidates, albeit all communists, for every election with participation levels by the populace exceeding any western nation by a wide margin.
Whatever piece of paper they finally wind up with or what they call it is irrelevant. It will be the implementation that will show the world exactly what form of government they have decided to give the Iraqi nation.