David: these are trees.
These are forests.
The fact is that nation-building always will fail. There are far too many variables in changing a football team's coach to make any secure predictions about a season. Imagine the far greater one of looking at a country's future after a new regime and attempting to see that?
The fact is that the US needs to meddle less in the politics of tinderbox regimes: how many times must we be burned to see it?
Petraeus is given huge kudos because of his successful policies in Iraq. But "successful" only lasted how long? Those who say we should have continued, indefinitely, to invest heavily in lives and dollars make the counter argument. 
As far as the shibboleth about a vacuum of leadership: bollocks. The US leads the world economically and that is where our strength comes from. Continuing to flush money on ill-thought out military adventures harms our leadership by crippling our resources.
Petraeus needs to shut up and disappear into a country club.