I've had some success using Confluence hosted from Atlassian. The front-end is well-polished and the document management features work well (mostly drag and drop). There's also a back-end API to extract JSON and XML data. Paying $10/month for the hosted service avoids having to spend time on maintenance. Confluence clearly doesn't meet all of the author's requirements, but it's a nice trade-off for someone interested in a simple pkb.