I recently read the new biography of Eisenhower by Jean Edward Smith. I think the experience of being a general in the U.S. military lends itself well to the presidency. Especially in Petraeus' case, as like Eisenhower, he had to manage a coalition of allies during a period of war, not to mention all the internal politics of being that high up the command structure. Leadership, ability to delegate and to choose the right people that can get the job done are all abilities we desperately need in the White House right now.
The modern president isn’t simply a manager. He’s a partisan who should be leading both popular opinion and Congress on policy. And if he (or she) doesn’t have a strong small-government orientation, the vested-interest establishment will steamroll right over him with ever greater federal government spending and centralization.