I like the Game Design Showdown excercise, and I think an equally useful excercise for designers could be to take a theme and write down three or four theme-mechanic links that show what aspects of the theme the mechanics will "simulate". (This is, in essence, what the GDS is, but I think the process I'm envisioning would be more concrete). When I'm starting a new project, I always look at the other games out there in the same theme/genre, and try to make sure my links explore a different "angle" than other games out there.