Murray has been knocking on the door of grand slam success for several years and should hardley be considered a long shot winner. Everyone knows Del Potro has the game: he just put it all together in 09'.
Tennis is a game of attrician and often comes down to who has more gas in the tank at the crucial stages of the final. One player has the confidence at the right moment and is able to take advantage of the moment when it counts the most, often one player is not. In Murray's case: he was simply ready to go one step further and was also prepared physically to do so (you saw Nole cramping, getting upset, ETC.)
It's true that probability is on the side of certain players making the final rounds of these tournaments because they have the most talent, are in the best shape, have the most experience in big matches, aren't afraid to win, ETC. So, we can with probability expect to see other players putting in the correct preparation and finding themselves in the position of Murray. At the moment Murray may have the highest degree of probability to continue winning these things given the fact that he has all the key components in place. Others may be lurking on the door step: time will tell.
2012 U.S Open Men's Singles Champion, 2012 Olympic Gold Men's Singles Champion, 2013 Wimbledon Men's Singles Champion...