While I like the idea of diversifying tags on games, thus improving the search system, the Steam user-tag system is widely abused and often inaccurate. Since they display any user tag once it hits a certain number of submissions, users can basically graffiti the Steam page with tags that don't apply, to the detriment of future users and the devs.
Perhaps users can submit tags, and once they're submitted a minimum of times, the dev gets a notification and can approve them if they apply?