Meh. Saul Alinksy's Rules for Radicals played as much of a role as anything. Media bias helped a great deal. The only thing I'd really fault Romney for is having a pretty bad ground game on election day.
There are some who think he should have gotten in the mud more, and it probably would have helped him, but he stayed true to he was. Not necessarily a recipe for success against a corrupt Chicago politician, but I like the fact he didn't get wrapped up in the sleaze.
And he was right about Russia (and many other things).