The yafaray glossy material shows a dark edge around it that gives a feeling that it's "drawn" and slightly like a "cartoon", reducing its realism.
For example, comparing this scene in yafaray with luxrender, in yafaray. Notice the dark edge around the ball, specially in the right hand side.
When rendering with LuxRender, it shows no dark edge, which makes it more realistic in my opinion.
I'm not sure if the problem is in the code of the material or in the algorithm itself. For example, I've been digging in Internet and it seems that Luxrender had, in the past, problems with the rendering of glossy materials:
They even had the hypothesis that the problem could come from the Ashikhmin and Shirley method. I don't know how they fixed that problem, or if it was really or not the algorithm.
Also VRay seems to suffer from this problem, for example in the next link, one of the parameters seem to be a "cheat" for the dark edges in glossy:
"Fix dark glossy edges - enabling this option will fix dark edges that some times appear on objects with glossy materials"
In any case, I believe that fixing this issue would improve the realism of scenes with glossy materials.
Best regards. David.