I think that S[O|F|U] should have moderator-only tags that get applied when a question is closed to help filter them out. These tags should be in addition to any tags (even if there are already 5) already on the question (I think that even closed questions should be well-written and properly tagged, but I'm willing to give up on this aspect).
On SO, this is a possible mapping between close reason and tag:
* exact duplicate => closed-duplicate * not programming related => closed-not-programming * subjective and argumentative => closed-argumentative * not a real question => closed-not-a-question * blatantly offensive => closed-offensive * no longer relevant => closed-not-relevant * too localized => closed-localized * belongs on meta.stackoverflow.com => closed-meta * belongs on serverfault.com => closed-serverfault * belongs on superuser.com => closed-superuser * spam => closed-spam
By default, these tags should appear on a new user's ignore list, invisible to users who are not logged in, and not indexed by search engines.