This is using default rendered view on the 12/15/15 WIP. I guess it is shadows being messed up somehow.
That’s the skylight shadows or ambient occlusion which is on by default in Rendered mode. The size of the object in relation to the file units plays a part in how smooth they look. Can you post the file? I’ll see if @DavidEranen can make it cleaner looking in a case like this… unless of course you like the effect in which case it magically becomes a feature.
The current skylight shadow algorithm has different levels of quality. Rhino can downgrade the quality if the GPU is struggling to render the effect fast enough.
It seems like in your case it was downgraded to Medium or Low quality. I agree with you that it looks really bad! In order to fix the problem I have removed the Low setting, and improved the look of Medium and High.
The changes should be available in the next WIP that is released.
-David