Every generation grows up with a new set of expectations. For example my parents had the expectation that they needed to buy encyclopedias if they wanted access to reference knowledge at home. I grew up with the expectation that I could get Encarta(tm) on a CD and later that I could search the Internet. My son is growing up in the Web 2.0 era where he can participate by submitting Wikipedia entries.
When it comes to open source the web site, source code, roadmap, defect tracking system, and forums are the 'project' and the community participates in the project. The fact that the source code and roadmap are available is a result of 'openness'. The fact that the defect tracking system and forums are available is a result of transparency. The fact that the design and initial code is available is a result of 'early and often'. It is these principles and the results of using them that ultimately attract and retain the community.
The community is a byproduct of the project. The project is a byproduct of the open source principles.
Participants in open source have an expectation of a community and the development model and professional open source company relies on it.