What are some fatal flaws in Happy Loman from Death of a Salesman?

The first fatal flaw of Happy Loman is that he is the son that most closely resembles his father, Willy. 

Happy is over-confident, unrealistic, shallow, and completely clueless about who he is. The reader wonders what is Happy's real role in the play, as his presence neither adds nor takes away from the plot. That is perhaps the entire purpose of Happy: He merely is. He just exists. He is a part of nothing. 

Happy (not happy)

There are good reasons for all of the descriptors above. First, Willy never really dotes on Happy the way that he does with Biff. To Willy, Biff (his first born) was the apple of his eye. Being the "runner-up" of the family, Happy overworks himself trying to please his dad. Since nothing that Happy does pleases Willy, Happy has basically decided that nothing that he (Happy) does, will satisfy him either. This is why, even though Happy has been able to hold a job, get a promotion, and move to his own apartment, he cannot make any significant connection with anyone. He is a lonely man. 

Another, very disturbing flaw in Happy is his blindness to the facts. Like Willy, he tries to live life in a formulaic way: Being happy = get the job, get the girl, get the money.

Still, none of these things ever make him sit and contemplate life, or what he has made of it. Happy is simply a living being that has not learned how to truly live. The worst part is that he sees nothing wrong with the way that he has done things. He even perpetuates Willy Loman's ridiculous dream and it is Happy who ends his part in the play by sticking to the idea of starting over in business again the way Willy did. Hence, Happy will continue the vicious cycle created the day that Willy Loman decided to follow the steps of  Dave Singleton, and left everything behind in pursuit of a shallow dream.