Hello,

All the users on your WP MU site have Subscriber level access on to all blogs on the site (i.e. they can comment on all blogs with single login). You can optionally change their permissions on a per blog level through the blog admin interface logged in as the site admin.

For the tags (keywords) you can use the wp-mu-sitewide-tags plugin

Lastly, for the site wide categories, you can do this - http://codex.wordpress.org/WPMU_Site_Wide_Categories - Although it's for the older version of WP MU.

Regards,
Arania