I have been using WP like MS sharepoint (for sharing both discussions and document libraries using categories) inside an intranet. A few features would make more appealing to sharepoint's users:
1. better search, particularly into the content of popular document formats, like .doc, .pdf, .mht, .zip etc.
2. GUI tweakable allowed upload file extensions
3. better or more flexible document folder structure for document libraries.
4. how about a new "Library" category that lists the documents within.