Jazzman asked if anyone could explain to him how the tags thing should work inside vanilla, then he will prolly make the extension. So here it is Jazzman. things i do for you :)
1) When starting a new discussion add a "Enter tags for this discussion" field as shown below
2) Once a discussion is started and tagged, A new section is added in the sidebar called "Tags for this discussion". Any user will be able to tag the discussion, either while making a comment, or by clicking on "Add tags" button.
The "Add tags" button is not shown but its under the "Your" tab in the "Tags for this discussion" sidebar. There is also a community tags tab in there. All tags added to a particular discussion will apear in this tab. If members posting agree on a tag, then that tag appear in bright color and larger font, meaning 9 out 10 people used the same tag. If they do not agree, meaning 1 out of 10 people used a particular tag, then that tag appears smaller and fainted. This way the community as a collective tags a discussion.
3) Members can only change there own tag list of that particular discussion. the community tag list is cummulative of all member tags of that particular discussion. this way members won't be able to mess with tags. If a member removes a tag from its list, All it will do is change the imprtance of that tag in the community tag list. That wills top epople from deleting tags and messing things up. One imp point. the tag importance is based on per discussion, per member. NOT per comment. That should also stop people from promoting wrong tags (unless they all decide to muck it up, then there is nothing u can do, beside warn them)
4) Searching of tags should show posts tagged by that word, plus on the sidebar it shows all related community tags as before.
5) Have Tag suggestion. For a particular discussion, for eg, Apple was used as a tag. next time if a uer types "Ap", it should automatically suggest Apple. The user can accept the suggestion by clicking the down arrow key, or continue typing. Just like you see on del.icio.us
6) feel free to add your own suggestions. I will keep updating this list, So Jazzman know what we want :)