I think one "feature" to stress is the user-focused nature of OpenACS. Most other toolkits are mainly focused on content. OpenACS has tools to manage content, but everything is derived from a user perspective. I found this description of OpenACS at a CMS information site:
Open Source web application toolkit designed for highly interactive user focused sites.
I would provide a link, but the site appears to be down. So I think all the bullet points should address how OpenACS fufills that description.
