Embedded Jetty (servlet container + Web server) or WAR distribution.
Embedded McKoi (Java database engine) Wasn't reliable, now any JDBC provider or just filesystem -- HansGerwitz (Now uses filesystem by default.)
Very easy to install
runs on Windows and anything else
UI is very clean and modern, including built-in themes
Clean and powerful syntax (including tables, code formatting, etc)
Support multiple users
Support a weblog style frontpage (create pages with dates as names, then they get listed in reverse chronological order on the front page)
Built-in full-text search
No revision control (once a page is deleted, it's gone forever) This has been addressed, but refactoring (e.g. renaming a snip) is still limited -- HansGerwitzAddressed, or fixed? 'Addressed' sounds like a workaround -- BinkleyBinkley / As of version 0.5.2a, revisions are saved and you can view diffs between versions. -- JohnHume?
No real backlinks support (backlinks are really HTTP referrers in SnipSnap), which means some wiki traditions like CategoryCategory don't work
Built-in themes' CSS not well-tested across browsers (lots of ugliness in MicrosoftInternetExplorer, though this is easily fixed from the UI, as all CSS pages are Wiki pages (or "snips", as SnipSnap likes to call them) editable by an Admin)