Just came across this one, which for some reason I had not seen nor heard of before:
I just hope the Boonex Community Software Experts didn't code their backend like they coded their html.
Drupal - plenty of modules that can do as you have described, not difficult to customise and a reasonably strong and supportive community (important). Some aspects need to be worked on, but these are being addressed for Drupal 6... my 2 cents.
I know cool cms Content management system, cms based on Ajax framework ! try...