McDowell won because he played smart. The 18th is the perfect example. Driver away from the Pacific. 9-iron (or thereabouts) when he knew he didn't need to make a 4. Wedge in. Two putts for a 5. Trophy.
Before that, he kept his wits when everyone else lost theirs, and, given that he was playing with Jack Lemmon -- er, Dustin Johnson -- that was a major accomplishment.
Overall, Round 4 of this Open brought with it the most amazing series of meltdowns since, I think, the 1975 Open at Medinah, which featured J.W. Nicklaus bogeying the last three holes. About a dozen guys left Medinah believing they kicked away the title, and the same was true today.
As Cary Middlecoff once said, "You don't win the Open. The Open wins you."