The ‘dark’ effect in the glass is due to the use of shadow-only groundplane. You actually see the custom reflection and refraction environment through the glass. I haven’t found yet a good way around it - I probably will have to adapt Cycles core to work for the Rhino usecase better. The use case being that through transparent objects one would see the background, but reflections still would see the custom reflection env. It is on my eternal todo list to improve the shadow-only gp, though.
Changing the groundplane to a white material makes the glass appear brighter. But the ground isn’t full white anymore, because it is shaded. Alternatively you can unselect the custom reflection environment, but you obviously lose the reflections from that image.