It reminded me of the film "The Pursuit of Happyness." You get drawn into the main character's misery. And then it gets worse and worse and you can't see how someone could survive under such trying conditions. Then it gets even more depressing, etc. Then, when the "payoff" comes and it is anti-climactic you feel like you suffered through all of the main character's sufferings and there was little reward at all. In "Pursuit of Happyness", after nearly 2 hours of suffering, you get about 30-60 seconds of the "payoff". I felt cheated. In "The Road", I kind of felt the same way. The ending was weak. Plus, the reader never learns what happened to cause the desolation, the scenario was not very believable, etc. Initially, McCarthy's refusal to use the man's or the boy's names was novel, but it became irritating after a while, and at times it was confusing trying to figure out which he or him said or did what. I really wanted to like this book, but in the end did not.
Different strokes I guess. Although I know what you mean I thought the ending was very appropriate. As for the cause I too would have liked to have known more but for me it was not that much of a distraction.
FWIW I agree with you about Happyness.