The tagging functionality is in my opinion one of the most potential features, that could allow MVCForum to set itself apart from its "competitors". The tagging system is what could be the glue that binds that different posts together in a "live" - wiki feed on a special "tag".
Who can create tags?
To keep the tags "alive" and not turn into a admin/moderator nightmare of keep the tags current, then I think that all users with X number of points/badge should be able to create new tags.
But I also like the suggestion by Bala that it should be possible for admins/moderators to change/approve new tags, and/or link them. As Bala, I'm also too familiar with the fact that not all users uses the same abbreviations or tags. Instead of just deleting/merging tags, then some of them you want to keep as synonyms, linking to the "approved" tag. And if possible not only admins/moderators, but also other users with X number of points. And why not let all users suggest or vote up/down on additional tags. All votes an activity in their points score and showing up on their activity wall (bringing even more "life" into the site).
Subscribe to tags
To keep the system consistent, you should be able to subscribe (follow) a tag, lust like categories, topics (and other users). But users should be able to decide if they want email-notifications or just getting the posts to the users onsite notification list (new suggestion?).
Should tags have a description?
YES! And a badge setup for who can edit them (plus admins). The way I would love to see it is similar to how Stackoverflow works: The description turns into a "mini-wiki" - being the glue that links questions/answers together with other questions/answers. Again putting too much work on the admin/moderators (for large sites), will slow down the growth of the site.
Which brings me onto...
Yes. Again badge setup for who can approve/delete/merge/synonym them.
Personally I have tried to get the users of my user group website to use tags for more than 10 years. And despite the growth of the #hash-tags, it has not yet gotten very popular. Only the more advanced users (they are not all techies spending much time in forums) are really using it.
So I would suggest the ability to have a list of required tags, where the user must add at least one of the "required" tags. In most IT focused sites knowing exactly which program or version of the program the question is about is crucial for other users to help answering the question correctly.
Tags and points (and categories)
When redesigning the tags setup, then I'll suggest that the points system is included in the redesign.
For adding real value to the tags and points, then these needs to be linked. When you get points for (correctly) answering a question, then these points should not just be "global" points, but be linked to the category, and the tags. This way you will be able to have "leaderboards" for the individual categories as well as each tag and thus being able to answering "Who is the expert in TAG-X?".