Many of the better Presidents we have had did not have military experience - Jefferson, FDR. And although Reagan was technically in the military, he was involved in public relations - so I'd keep him with the non-military folks. Also, LBJ was not in the military, but he is probably in the middle. Then there are those who WERE in military that I can be classified as some of the worst - Grant, Nixon, Carter, and yes, George W. Bush.
The framers of the constitution want to make SURE that a civilian was in charge of the military, not a career military man. And yes, there have been military presidents, but all of them resigned their commissions when they became President. The framers did not want to the US to fall into a military dictatorship.
I don't think military service should be a simple yes/no answer - if the President choose the right advisors, he can be a pacifist and still be an effective commander in chief.
With Obama's election, I've heard that this will finally "end" the Vietnam war. It's very likely that whatever candidate the Republicans put up in 2012 he or she would have likely only had peacetime military experience, if any at all.