I think that tags with which everyone can contribute is tricky, tags usually seem to work better if you can have your own taxonomy of tags and they contribute to the site. (eg: delicious lets you tag stuff, and browse those, in addition to everyones tags.) With an implementation of that scale, I can think of more valuable features to implement (like combatting the decline in comment quality recently)
If there were a checklist of guidelines about comments and you had to physically check each one to post a comment, it would A) reduce trivial posts by making them more work to post and B) provide reminders -every time you post- about the kinds of content desired for posts.
Easy fix: simply impose a timer on adding comments. If you're only allowed to make one comment per story per 5-10 minutes, you'll choose more carefully where/when you place comments.
Oh, and even if there is a GreaseMonkey script, displaying reminders is still a good idea.
Really not sure what tags would achieve over Google and/or http://searchyc.com/