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.
