What if WordPress allowed you to have a demo-content.xml file in your themes directory so when it's previewed you can see the theme how the Author intended, an added bonus feature would be when you installed it, it could setup the theme also how the author intended. Also an easier option would allow theme authors to op-in to hosting a demo theme and allowing it to be viewed in the preview.

@Jeffro - I built ExperimentWith.com for all kinds of WordPress testing so if you ever need it let me know, We are willing to use it for all sorts of projects for the community.