We've been using FogBugz for tracking bugs and features for some time now, but are just getting started with wiki, and it's great!
One feature which would really help would be for searches (using the good ol' search box we use all the time) to also search within the content of attachments, particularly Word documents on Wiki articles.
The reason for this is that we want to start keeping our library of requirements documentation as wiki articles and they often get written as Word documents, plus we have a tonne of them already. Having them as searchable content without the need to copy & paste them into wiki articles would be really helpful.