Anyone who has used SSRS in the past has probably been slightly frustrated with the lack of control for parameter positioning. It was possible to have some control by manipulating the ordering of the parameters, but for 2016 we have a new interface to define the positioning. It’s basically a grid onto which parameters, along with their labels, can be placed.
The big one for me: HTML 5 support. I remember spending so many hours trying to figure out why reports wouldn’t display in Firefox correctly or why they sometimes wouldn’t work at all (because the report builder executable wasn’t installed correctly or that one time there was a bug in the executable)…and that was before mobile took off as a serious platform.