Why does people seem to think that war heroes will do great in elections based on their military record. Where is the evidence to support that claim? Let's look at the last five US elections:
1992: Bill Clinton beat George H.W. Bush
1996: Bill Clinton beat Bob Dole
2000: George W. Bush beat Al Gore
2004: George W. Bush beat John Kerry
2008: Barack Obama beat John McCain
Notice a pattern? Each and every time the guy with the superior military record lost. The theory that military service matters in US presidential elections would suggest that two term presidents like Clinton and Bush junior never stood a chance.
Even more interestingly, during the last two presidential elections, the candidates were relying heavily on their "war hero" status to secure their party nomination. Somehow the parties think that being a war hero makes you more electable.
Sure, being a war hero can't hurt you. It WILL probably benefit you. But politics and communication skills matter so much more. When the parties get blindsighted by a person having war hero status, they tend to forget to concentrate on what really matters. While I am personally a huge fan of John Kerry, his ability to connect with voters was just in a different (lower) league to Clinton and Obama.
I think you need to look at a General or an Admiral differently than a politician with a war-story. The reason why Petraeus is so impressive is that he had a unique plan for a desperate situation which turned out to be relatively successful. While a Bush/Dole/Kerry/McCain war-story speaks of the man's character, it does little to show his ability to formulate big-picture ideas and delegate authority to implement those ideas, all while at the mercy of Congress. Quite similar to the Presidency.
That said, I have no reason to believe Gen. Petraeus will ever run for President.