Ruthlessness dominates ISIL tactics
The leaders and fighters lack inhibitions when it comes to war crimes. This ruthlessness tends to frighten many of the people they face in combat. They also have become somewhat proficient at maneuver warfare and combined arms operations to a limited extent. This allows them to overcome lightly armed opposition. So far they don't seem to have an anser for US air attacks other than to murder a prisoner.
As fighters continue to seize territory, the Islamic State in Iraq and Syria has built an effective management structure of mostly middle-aged Iraqis, including many military officers under Saddam Hussein.