Post History

The sole purpose of tags should be to help classify posts into what they are roughly about.

Being able to easily get the broad topic of posts is obviously useful in searches, but may also be useful for statistics in the future.  For example, if a large fraction of posts are about a particular language, then it might be reasonable to create a category just for that language.  Such a query would be hard to do without good tags.

Overloading tags to provide anything other than a broad classification of topics will lessen their usefulness for the intended purpose.

When deciding on using or creating a particular tag, a good question to ask yourself is <i>"Will the tag help people find this post when searching?":</i>.

Tags are also not content.  Just because way down at the bottom a post is tagged "C++" doesn't absolve the asker of mentioning the language is C++ in the question directly.  The text of a question needs to stand on its own, regardless of what is in the title or the list of tags.