Wiki's allow content to be edited easily by anyone. At least at the top level. The most famous example is wikipedia.org which has its own entry on wikis.
From a Tendenci perspective, the main point is to observe that users change their behavior based on how easy it is to make an update. So in the past they might have said "hey, I get great service when I do my monthly updates" we see them instead to 10 or 20 monthly updates when friction is removed.
Additionally we believe users are highly capable, more so than imagined, and provide SEM tools for the *end user* to optimize *their content*. They are after all the knowledge matter expert. This is done in the form of search engine tools that can be used to evaluate exactness of speech.
Feedback loops are a simple concept. Ask the users about the content. When rating content the author has an intention, the user understands something which may or may not have been the author's intention, and then there is a desired result. For the result the question is "will the reader change their behavior as a result of this content?" 99.9999 percent of the time behavior won't change as people like to read what they already agree with, but there ARE exceptions and feedback loops provide these. Feedback loops ARE in Tendenci in the form or ratings and object-contact-forms, like the contacts in directories or articles that go to the owner of the content as opposed to the site owner.
In practice I have found feedback in the form of ratings to be negative. Which is a bummer, but they are telling us how to improve. This guy apparently did not like the quotes search functionality and now intends to use the crapper. Good for him. http://www.schipul.com/en/ratings/view.asp?ratingid=18
Tagging is the most fun and is visible on the following links - note the size of the text on the following links indicates the approximate number of tags. So BIG WORDS have more activity.
and here is a similar page on flickr
and here is music tagging from last.fm - different view from the others
Think of tagging like categories, but instead of being limited to one category and one subcategory, you can have unlimited categories in the form of tags. It is an approach to solve the problem of diverse topics. So one photo might be tagged family, woman, love, wine, sunset, vineyard - those are all valid tags for a photo if that is in the content. It is a complex problem and tagging is a simple solution to it. Clay Shirky has an interesting post called "Ontology is Overrated: Social advantages in tagging" that ties some of these concepts together. And the BBC did an interesting presentation on music and social software with tagging at the eTech conference earlier this year. http://www.oreillynet.com/pub/wlg/6701
What makes tagging so interesting is SHARING the information with others as both Technorati and Flickr do. Tagging is by definition distributed authoring with strong subgroups. Tagging is not yet in Tendenci.
New topic. Trackback is a way to comment on other people's content but keep the content in your domain. So if you post a comment about someone else's blog, should you really have to then post your thoughts on your blog? Will you? Probably not. So with trackback you can say I am commenting on THIS content so put a little link there but have that lead back to my blog with all of the rest of my content. Trackback is a bit harder to wrap your brain around but is explained here: http://www.movabletype.org/trackback/beginners/ Trackback is not yet in Tendenci.
For more on the problem of social software, Shirky has a post that is helpful http://www.shirky.com/writings/group_user.html The catch is, in my humble opinion, Shirky leans towards "online communities". By that I mean he likes to look at communities that formed online, like Slashdot.org. We have a more complex problem in that our software must interact with communities that were first OFFLINE and are now moving to a hybrid of ONLINE/OFFLINE. Membership organizations, associations, registered site users, anonymous visitors, search engine robots, this is a diverse audience and we are working on a complex problem. But it is damn cool and www.tendenci.com is making great strides.