here's my quicklist on the tech. side (I'm biased towards the wikka
engine, http://docs.wikkawiki.org/WikkaFeatures):
- ability to extend the markup through "actions" and "formatters"
(see http://docs.wikkawiki.org/UsingActions and http://docs.wikkawiki.org/FormatterInfo). this allows integrating
tracker, docbook, cvs, anything.
- free/open source and written in a mainstream hi-level language
(php, python, java, perl).
- essential features: discussion page, smart title, auto linking,
interwiki linking, editor toolbar, image links, tables, file
attachments, code highlighting, delayed indexing, diff revisions, text
search, changelog, mass page reversion, refactoring, notify changes,
link checker... see http://wikkawiki.org/CodeContributions for other.

I have a 24h linux server for hosting but it's down here in Romania on
a no-SLA ISP, so I guess I'll skip on offering it.

my2c.