Of the Virulence and Infectivity of the Influenza Virus
There is a reason of this major difference between H1N1 (and, for that matter, most other strains of influenza) and H5N1. It all depends on the locations where the different strains infect. In the case of H1N1 and most other flu viruses, they target the upper respiratory tract (e.g. the nose, throat, and upper trachea). This makes it easier to spread, for being closer to the outside world, but less virulent for they do not cause pneumonia in the lungs.
H5N1, though, targets the lower respiratory tract (the lungs). When the immune system detects the virus' presence, they pour into the bronchioles and try to consume and defeat the virus. As some of the white cells die, they form pus, which results in the deadly pneumonia characteristic of this particular strain of avian flu. However, their location deep within the lungs makes it hard for them to escape.
The aforementioned difference of target location is the main factor between the virulence and the infectivity of the different strains of the influenza virus.