Sharing issues over oil, as I said. One Shia state with oil initially, and one without. I expect those will eventually merge but the delay will give the Sunni Arabs enough time to create a viable state of their own, almost certainly centered around the rural tribal leadership.
I had thought since 2003 that the Shia would eventually run the Sunnis into Syria, but the experience of the Maliki government shows the Shia simply lack the organizational ability to project the necessary force.