The revision system introduced in WP 2.6 allows for a neat new feature: stable versions of pages or posts. Two things are needed:
1. the ability to flag a certain revision as the stable version
2. an interface to easily revert to the stable version
Note that this is not a request to make WordPress more like a wiki. Rather, this would be a feature that would make it more useful as a CMS. I'm thinking of clients who want to fool around on pages, but with the assurance that they can safely revert to a stable version if they mess up.
Basically, stable versions are privileged revisions. This would be an important improvement to the current flat system where it is not even possible to see at a glance which revision you'd want (because of the lack of edit summaries).