An austrian educational instituation has conducted an Open Source CMS comparative study. Here are their criteria, and recommended products (Typo3, Plone, and PHP-Nuke):
http://virtual-learning.at/cms/mindestkriterien.htm
http://virtual-learning.at/cms/ergebnisse.htm
Looking at these systems and criteria ought to be useful if we are to revitalize OpenACS CMS.