First one that comes to mind -- it kills the hoary myth that conservatives steal elections and undermines what little credibility the claim has for elections in the past.
Secondly, that a man of color named Barack Hussein Obama was elected says something magnificent about America, how this truly is a land of opportunity, a meritocracy where talent and the ability to articulate a vision of hope matter to a great many people.
If I had to pinpoint the main reason McCain lost, it was his hopeless incoherence, especially after the financial panic hit in September and we were hungry for steadiness and clarity. Obama projected both; McCain, neither.
Third silver lining -- many people on the Left would have refused to accept a McCain victory, regardless of whether it was disputed. What followed may well have been '68 redux.
Lastly, I doubt we would have gotten three consecutive terms of Reagan/Bush without the disastrous four years of Carter that preceded them.