Which tags do we allow?

Just everything

This is what everybody does right now. Just every combination of letters will be valid as a single tag.

Problems:

Different people might understand different things under the same terms, but maybe this is irrelevant if you search for more than a single word?
SPAM ..

Allowing only words that are title of a Wikipedia-Site, or at least use wikipedia as follows to improve the quality of tags automatically:

This could solve some problems:

SPAM
Words that are acronyms with several meanings could be solved via the information that is contained within wikipedia already. (e.g.: http://en.wikipedia.org/wiki/DRM) The tagging-interface then could automatically provide a list to the user and allow to choose one of those meanings (by replacing the tag with the correct wikipedia article name).
Each tag has a described meaning, thus people will more likely understand the same
A search through tags would provide the searcher the same lists for which meaning (of such a word) to search.
Often there are several words that mean the same thing. This can also be automatically solved with the help of Wikipedia: Redirects. Redirects are Wikipedia-Sites that contain nothing but a reference to the article with the correct lemma for its meaning. Such tags could be automatically replaced with its correct lemma. (e.g.: http://en.wikipedia.org/w/index.php?title=Asymmetric_Algorithms&redirect=no)

Problems:

Lots of other good ideas about a Software Thesaurus can be found here: http://www.groklaw.net/article.php?story=2006052522234947