I use News/YC on my iPhone. Commenting is broken so I couldn't even if I wanted to :) The story is a little different on my laptop (as you can see) but usually I only let myself comment if what I'm going to say will add value to the discussion: a new perspective, a yet-undiscussed piece of information, some well-reasoned advice (and this only if I've been in similar situations), etc. Also, if I see a subthread between two people that is clearly going nowhere, I avoid it.