It's a little bit of both. New England has one of the best quarterback & coach combinations of all time. You'd really have to go back to the Joe Montana & Bill Wash days with the San Francisco 49ers to find a similar tandem.
In most years no one would consider Denver with it's 8-8 record going against a team that is 13-3 and at home (healthy and rested) plus a wealth of experience winning the big game would have any chance of being upset.
A couple of lucky/Forrest Gump events and the Tebow Express was roaring down the tracks. America loves an underdog. Last night's game was a reality check.