Not to take anything away from all the nominees, because they are all good actors, but when you look at character development, Johnny really did the best job with Captain Jack Sparrow. How many people who have seen the movie will ever hear the word "pirate" and not think of him?
I think comedies are overlooked a lot of times because they are comedies. As Bill Mrray has said "People,in general, are a lot closer to crying than laughing". Unfortunately, it's usually a big drama that will win the awards.