A few weeks ago Chuck Frey of the mind mapping software blog asked people to share their thoughts on how they would define a great mind map.
There were many insightful comments that covered a wide spectrum of what makes a great mind map.
Chuck did a follow up post with a summary of the comments.
Jim Deiner, one of the participants, even made a mind map of all of the comments - available as a pdf or as a MindManager file.
I would highly recommend you check out all of the comments and the summary to get a good snapshot of what people think makes a great mind map.
p.s. I am comment #2