There are a few differences between the book and the movie.  The first difference that I noticed right away is that Hassan does not have a "hare lip" in the movie, which I feel is quite a significant idea in the novel, since Baba pays for a surgeon to repair it.  This, of course, might be a bit difficult for the make-up artists to accomplish.

Also, the movie makes it appear as if the adoption process was easy to accomplish, but in the book it is a long, difficult, and arduous process.  And if I recall, the movie also does not show Sohrab's suicide attempt.

In going for an obvious answer to your second question, perhaps Khaled Hosseini thought the novel would make a good movie simply because this was his first novel:)  Aside from that, the novel does provide a powerful example of current events: modern-day Afghanistan, the Taliban, the Russian invasion, etc.