It's interesting: Those Wikis turn out to be the best (regarding usability) which call temselves not even so. Like 
last.fm's bio page or Google Groups Pages. I think with Wikipedia's dominance everybody now believes that Wiki=Monobook-Theme. I beg that we get rid of this cluttered beast. I see Wikis more like frameworks. The theme is just the surface. Monobook as a surface does a good job in hiding what the original idea of Wikis was: Speed. And ease of use.
As Wiki now stands for Wikipedia/Monobook, it's not a bad idea to call a service different. The Wiki-Idea stays as a concept and does its core tasks under the hood (versioning, user management).
This is where PhpWiki comes in. PhpWiki is kind of an underdog. The default theme lacks the feature bombardement of Monobook and is not quite appealing to the eye (like, say WordPress - I know, it's not a Wiki but that's a different (long) story). I think the lack of a good default theme turns many people off. However, IMHO it has a fairly good if not exceptional codebase. When I think further and look at the latest PhpWiki developments regarding Semantic Web and Wysiwyg, what I see is not just a plain Wiki but the basis, well, a framework for building future web applications. Maybe we'll just have to name them different. Like SocialText, SystemOne, JotSpot and others of course.
Google Groups Pages via: del.icio.us/gregorrothfuss
btw, this page was made using PhpWiki too.