Is the main difference between graffiti and street art the intention behind each approach? Graffiti writers are not interested in the general public understanding their artwork. They are primarily concerned with other graffiti writers who can decipher the coded tags and appreciate the writing style. This ignores the intended public impression created by graffiti which can be complex.
To assume that street art is intended for a different audience still leaves the question of whether it is art? Perhaps it is enhanced skill, but then it is likely in the eye of the beholder to decide.