I just saw the movie, and I was disappointed at the end.
This movie was fun, it was entertaining, and I suppose it has done its job, but it was not supposed to have been called 'I am Legend'.
Robert Neville was a legend, because he was what every infected (vampire) person in the world feared.
As he used to go out, killing them every day in their sleep (as they had aversion to sunlight, and other things vampires traditionally had an aversion of, mirrors, garlick, stake through the heart). They feared him more, because he could walk in daylight, when they couldn't and murder them one by one.
He became to vampires, what the vampires were to humans.
Hence Neville was a Legend.
In the movie, I did not really know why Neville was a Legend. He was more a survivor, convinced that he can save Everyone.
Spoiler:
Neville's death in the book was what made him a legend - Neville's death in the movie was just too mindless - he died for the sake of dying