Notes:
How do we go about building web sites that let users participate and collaborate? The basic open source stack is already available in the form of Linux OS, Apache web server, MySQL db and Perl, PHP or Python. Ok I can see some Ruby enthusiast cribbing; Perl, PHP, Python or Rails. You'd probably need a wiki based content mgmt system or a blogging software depending on your application. Actually, the distinction between blogging software and wiki software is getting blurred. Comphrehensive CMSes like Drupal and MediaWiki support both applications. Decide what features you need when you make a choice.
Tagging is generally useful to organize content along multiple lines. You might want some sort of comment moderation in the form of Slashdot moderation ratings or captchas (completely automated public Turing test to tell computers and humans apart) to prevent comment spam. RSS feeds are generally a standard feature of these tools. It lets pull whatever content they choose into their RSS readers where they can read it along with other content from different sources. Permalinks allow others to link to a blog entry without worry that the link will break with the chronological progression of the blog. Trackbacks let you know when others link to your blog entry.
The key feature in a wiki is support for revision control. This is the primary safeguard against content mischief. Some wikis also support attachments which could be useful if you are trying to aggregate multimedia content. Some people are not comfortable with a free for all edit policy, so products like WikkaWiki provide access control lists.
So much for what can be achieved with freely available software. For other aspects like facilitating mashups and usability we need to do a little more.