Mostly dittos on what Drew's saying.
A few things in TWiki's favor:
- Automatic linking of [link|#|WikiWords]. For building references, this rocks.
- Moving and renaming. You want to change a link? Every single page that links to it is updated. This isn't something you can appreciate properly until you've maitained a site through at least one revision cycle.
- Search. It's built in. It's both simple (the search dialog on every TWiki page a [link|http://twiki.iwethey.org/twiki/bin/view/Main/|TWikIWeThey] are something I added to the default template -- a 60 second hack -- but you can search from any page) and advanced. The [link|http://twiki.iwethey.org/twiki/bin/view/Main/WebSearch|Search] link gives you a choice of simple or complex (regexps!) search.
- Multiple navigation paths. You can come in via the [link|http://twiki.iwethey.org/twiki/bin/Main/|front page], [link|http://twiki.iwethey.org/twiki/bin/view/Main/WebSearch|search], check [link|http://twiki.iwethey.org/twiki/bin/view/Main/WebChanges|recent changes], or a [link|http://twiki.iwethey.org/twiki/bin/view/Main/WebTree|tree view]. Flexible, and suits many browsing styles.
- Simple syntax. Straight text, with two carriage returns between paragraphs, is all you need to know to write in a TWiki. If you know HTML, use it, though the simplified markup syntax is IMO easy to use and generally clearer.
- Automation. Searches and other "TWiki Variables" automatically generate content, including lists, directories, tree views, and related data. Data need only be entered once to be referenced elsewhere in the TWiki. You can also extend the underlying scripts as you desire. Plugins make for ready additional extensions. And the InterWikis plugin allows Wikis (and many other web-based systems) to be reference from within a TWiki.
- For the asthetician, there's skins. For some different views of what TWiki can look like: [link|http://vmlinux.org/twiki/bin/view/Main/WebHome|vmlinux], [link|http://friends.mannby.com/bin/view/|mann by], [link|http://twiki.selfish.org/twiki/bin/view/Nob/WebHome|selfish] (a personal favorite), and (an exciting discovery) [link|http://eon.law.harvard.edu/twiki/bin/view/Openlaw/|OpenLaw] by Wendy Selzer.
Answering Drew's comment about document storage format -- TWiki uses flatfiles in a directory. So exporting topics is pretty damned easy.
Sequential/temporal discussion is somewhat counter to TWiki's goals, but it
does have means for seeing topics which are under current discussion, which may be of more interest. There are also methods for aiding generation of temporal content, though these tend to be (IMO) clunky. Then again, temporal discussion is often clunky, particularly when you've got to deal with lots of it at once -- TWiki's worth definitely grows over time.
There are simpler Wikis, including some which are merely a single drop-in module/file. But TWiki's toolset is quite nice.