I agree with all of Mark's suggestions. Here are mine (in order of importance):

• The ability to upload read only files such as specification docs - this would remove the need for our documentation repo, which is quite hard to navigate (or even know what's there).
• In keeping with the spirit of Adapt, I'd like to see us adopt something that's mobile-friendly.
• In addition to images, would be nice to have support for other media (video, audio etc.).
• Theming options (to fit the Adapt branding/community site).
• Any integration with our existing tools/processes is always a benefit, especially git/github and JIRA.

FYI: GitHub's wiki engine is open-source: https://github.com/gollum/gollum (and ironically, I believe it has search). GitHub's wikis are awesome because they're git-based, so you can manually check them out and tinker around with the posts offline.