Personally, I try not to spend too much time on HN (sometimes I enable noprocrast), so I need to quickly decide whether it's more worthwhile to read a single ten-paragraph comment instead of ten shorter ones presenting different points of view.

Usually, if the first paragraph is not absorbing enough and there is a risk that the rest is going to be rambling, I simply stop reading without any regrets. For me it's not an attention span issue, it's a limited time issue.