The good ones are able to catch problems before stuff is coded or modeled. A good designer manages to describe every object needed (including rough sketches what they should look like and describing needed animations), every mission condition (which is very close to coding and needs good understanding what is technically possible). A good designer can describe the GUI pretty much ahead in a way that coders don't have to rework it over and over until it fits (including describing stuff like left-click/right-click/drag&drop/selection box behavior which can get very tricky pretty fast in games). A really good designer might even know excel good enough to calculate all kind of stuff through before even approaching any coders.
A bad designer thinks it's all just about some great idea and throwing in lots of stuff and the team will do the rest. A good designer knows it's 90% about describing details and about removing as much stuff as possible.