Oh Glenn, why so cynical? It's almost as if you've done this sort of thing before... (^_^)
If the wiki supports hierarchical namespaces (which are handy, but not essential) then I suggest checking if it makes it really easy to move pages around to different namespaces. Otherwise the self-organising nature of wiki becomes somewhat harder. It's less of an issue for flat wikis, although these do require people to pick ReallyDescriptivePageNamesThatAreUnlikelyToClashInTheFuture.
Dokuwiki, for example, supports namespaces really well, but moving pages is a pain (basically a manual cut-and-paste job and the history of the page isn't copied across). This is fine for a tightly-controlled wiki, but if you're going to allow people to create whatever pages they fancy then things can become disorganised and this makes content discovery difficult. I'm afraid I don't agree with Google's "search is all you need" mentality when it comes to documentation because often it's hard to guess appropriate keywords until you know what you're looking for.
Of course, the principle is that the users of the wiki will write their own contents and index pages on demand, but again, this behaviour doesn't tend to develop totally organically, it needs a little encouragement.
Anyway, just some things to bear in mind. Picking a wiki engine always seems to involve compromising somewhere, so it's probably not worth being too picky. Unfortunately I'm the picky sort... Good job it's not me doing the choosing! (^_^)