[juanmanuel]

And good modelling and uvmap layout helps a lot too. It could be you have a wonderful lighting, but the silhouette of the shade looks faceted instead of smooth. Or you have a perfect shape and lighting, but have the textures stretch out unnaturally.
And remember, in the real world, almost nothing is either completely smooth or completely sharp. Or completely clean or completely dirty.
I think it is a combination of a lot of things.
But as Thomas says, if you submit your image to the forum for critique, you'll get valuable feedback as to what many people have to say on improving your image.