Possible Solution: An app (or a browser addin/plugin) can be created for filtering comments. App can process all the comments on a webpage and filter them in 4 major buckets namely appreciation, critique, 'related info' and spam.
Appreciation bucket can club all the congratulatory comments; critique bucket will contain critics’ comments; spam bucket will club junk comments and totally non related info, and 'related info' bucket (the most important bucket) can contain comments where other users have added their own opinion and extended the discussion to higher level or some user commented other contextual useful info.
Let’s give people whatever they are looking for. In today’s Google-world when people don’t want to waste even a single second in scanning non-useful information and search even for smallest thing, comments should also be filtered to let people read whichever bucket they are interested in.
Once we started filtering comments this will impact the comments writing behavior as well. People interested in the real discussion will feel encouraged and will participate more as they will know that their opinion will not mobbed.