The current tagging system has a lot of freedom for the user. The disadvantage is that the tags become a mess quickly: every user defines his/her own. There are many (almost) duplicates, different names for the same, etc. And the taggign becomes unusable. DNNSoftware.com is a good (?) example.
Suggestion: suggest relevant (existing) tags based upon the content. Use the new help indexing to find content with similar content and use the tags of that content as automatic suggestions.
This will increase the value of the existing tags and disencourage the creation of new tags.