What if light had a rest mass?

How different would the universe be if light had a mass?
would some physical laws be changed?
and how would man be living differently if life was still possible

Hypothetically, the speed light of light through a vacuum would be slower and cease to be constant.  Not to say that the mathematical constant 'c' would necessarily change; light would simply no longer travel at it.  I also imagine it would lose all wave characteristics (diffraction, interference) and behave more like a "pure" particle.