Over the past few months I have talked quite a bit with Joel about online communities and about what philosophically makes a community. I think it is a group of individuals who collectively are interesting in something and need tools to facilitate communication and production of some artifact(s). The way the community interacts to make this happen differs greatly and so thinking that one tool set will cover all needs is short sighted.
Now I go back to being a WordPress fan boy. I think one of the big reasons WordPress is so successful is that not only is it open source but it is open source that is easy to skin, hack and extend. It is a platform that provides some really great functionality, but the real power is that it empowers experienced and inexperienced hackers alike and gives them a powerful place to begin building something that fits their needs.
WordPress is hackable. Communities have specific needs. Build the basics that are required to fit those needs and then let each community build their themes and plugins. In all likelihood the vast WordPress community will build thousands of templates and plugins that meet their needs and the needs of the thousands of communities that spring up around the first truly open source community platform.