I can't say that I have any hard or fast rules. Generally, though, I do like to see people's reactions to things, particularly on controversial topics. It's true, many places have low quality comments, so if they're particularly vitriolic, I might only scan a few top comments to get the gist. I like to play devil's advocate sometimes (often to my friend's chagrin) so if I can find an argument or line of reasoning I hadn't seen before, even if it is one I don't agree with or could poke holes in, I feel like I gained a little bit of value.

I think an important distinction to make is the difference between comments and discussion. Commentary implies a kind of one-way offering of one's opinions (obviously attractive to many people) whereas a discussion requires that and more. The person offering his or her opinions has to be open a critique of their own ideas; as much as you may want to bring the other person to your side, so-to-speak, you also have to be willing to let them bring you to their side. It's hard, and more rare, because both parties have to make that effort. It's much easier to just give your opinion and check-out, mentally speaking.