On Tweakers you (as a user) can apply for mod rights and it allows you to rate each comment from -1 to +3.
+3 = must-read
+2 = informative
+1 = on-topic
0 = off topic
-1 = spam / insult
There's an alogrithm that gives your rating more weight if they (retroactively) allign with what the supermods (employees of the site) rate comments.
On Tweakers you almost never read insults and "and my axe" repititive comments because they are hidden by default, and the +2 and +3 comments automatically get places at the top.
This has so much over a simple +1 / -1 system. I wish more website would think about their comments systems and how they can achieve a good incentive and moderation structure for informative comments to be lifted up and unwanted ones down.
Tweakers is Dutch but to get an idea here's a news thread with a lot of comments: