It's possible/likely this has been posted before (I did a cursory search but didn't find anything) but this list is one of the great tools for game design: www.8kindsoffun.com
Having something as simple as a list of *different* notions of fun (and remembering that each human being weights them differently in their mental rubric of "what is fun") can be really useful in identifying what is "missing" from a game concept (or conversely, identify potential overscoping if you are trying to hit too many bases at once).
