An alternitave method to tackle the abuse of mistagging a post is to have the tags weighted, and the readers determine if the tag is relevant or not.
For instance say I tag this post with #feedback, #funny, and #offtopic. Each one of these tags gets a value of 10. Within the post along side the tag with be a small '+' button and a small '-' button. These buttons affect the weight of the tag. So when a reader finds that a tag has nothing to do with the topic at hand they can click the '-' button and lower the weight of that tag. If eventually the weight falls below a threshold, then that tag turns red, and basically becomes a NULL tag. Also the opposite could be true, if enough people think that a post is perfect for a tag, then the reader can click on the '+' button to add weight to the tag. After enough people "bless" the tag, then could turn a gold color.
It could also work that some of our more trusted members affect the weights differently than a regular user. * points at ninjas *
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A problem I see is, what about a user editing and modifing the tag... hmm..