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There are a couple of ways that I could see rating tags coming into play. One would be to give posts with these tags more 'staying power' in your feed (and possibly popular), and the other would be to favor posts with these tags in external posts. Both would have the effect that you would be more likely to see the best posts with that tag. I can appreciate the effect of not following every post with a specific tag (therefore avoiding the a subreddit-like phenomenon), but biasing a user's discovery with certain tags in mind.