You can be more than one thing. You can be an expert in conventional and unconventional war. Officers and senior enlisted of high caliber and flexible mindset are what is needed. In the Marine Corps we have legendary leaders who started out in irregular conflicts and rose to great success in conventional wars. Smedley Butler, Chesty Puller, Dan Daily, to name but a few. Warfighting can be complicated but it is not so difficult that it cannot be understood on its many levels. My opinion on the choice of Petraus is that he is intelligent, talented and he has an eye for spotting talent. He is the flexible officer capable of adapting to his environment. If the Iranians stormed across the Iraqi border in waves of armor and infantry I expect he would deal well with that just as he is dealing well with COIN. Placing a top notch GO in charge of a board is the right thing to do.