Holden and Novak miscast
Its already been stated that, at 37, Holden was too old for the part, a down on his luck, ex-college jock. Also, as an established leading man Holden would bring (or at least the audience would expect him to bring) leading man characteristics to any part he played. These characteristics would be strength of character, toughness, intelligence, maturity, authority, etc. These are not the characteristics of the character Hal. Like Holden Hal is good looking and athletic. Hal is amiable and people like being around him. However, Hal is also a broken person. He has the sense that he is inferior and trys to overcompensate. Although not retarded he is not too smart. He lacks the resolve to see things through such as finishing college or sticking to one job or living in one place. It would have been interesting to see Ralph Meeker who played the part on Broadway (although by 1955 even he may have been too old) play the role in the movie.
For similar reason Novak was miscast as Madge. No doubt she fit the bill as the pretty girl who could easily be crowned Miss Neewallah. Madge is a character similar to Hal. Madge is insecure, immature and perhaps not to bright. Someone with an agreeable personality but not too strong. Someone who has succeeded so far because she is pretty. Novak was comfortable with her beauty, intelligent, and secure in who she was and I think these qualities came through despite her trying to act the part.
Perhaps someone like Marilyn Monroe could have played the role of Madge better. The ending may have made more sense if other actors brought out the flaws and vulnerabilities of Hal and Madge. They were much alike and it was natural they would fall for each other. Some don't like the ending, asking why some like Madge would run off with someone like Hal. Maybe its because Holden played his part a little better. They're really asking why would someone like Kim Novak run off with Hal Carter.