MediaWIki as seen in Wikipedia has some nice features that enhance user interaction - such as random page and backlinks ("what links here"). It also supports unicode, which for some reason WTH's implementation does not seem to do.

Didn't see something that filters twitvid or videos?

If What the Hashtag had an account that auto-twitted definitions every 5 days then Twitter searches would yield definitions of the hashtags as part of the search.

(alternatively your could simply tweet the hashtag with a link to the wikipage with its definition).