CMS for documentation

Hello fellow Daniwebbers, I have a question about choosing a CMS. I know what you're think "OMG another 'what cms' thread". Hold yer ponies because I want to be very clear and precise. Having worked with multiple CMSs over the years, I have acquired a knowledge of which CMSs are best at what. Wordpress = small sites/blogs, Drupal = anything, and joomla = small sites/news sites.
What I want to do is create a cool site where users can just post documentation and tutorials on specific subjects. I don't want it to be a forum, just a strict documentation center. Users can all collaborate to make tutorials, but in doing this I need a strong taxonomy system (like Drupal) where I can sub categorize, and tag potentially millions of articles. Needs to be VERY lightweight and SEO friendly. I have used Druapl many of times and it is just the most complex mother ______ing CMS I have ever used. Any suggestions that re not Drupal? Example modules/plugins would be great.

simple list of requirements in case you didn't read above...No Drupal
strong taxonomy
simple
lightweight
SEO friendly
example modules/plugins in reply
easy to theme (optionall)