I try to read the original post and reply to that person only or, in longer threads like this one; I might read more of the original poster's responses, to see if the original poster has moved further on in their thinking or circumstances but I think I am often tempted by the "subject" and other responses rather than the original person's question or concern.

I think in longer threads things can get muddled and those who don't read to get a sense of where the original poster is can confuse and create difficulties for all readers by responding to other than the original poster (and not making that clear) or responding to something in themselves that the subject matter or other responses elicit which isn't germane to the original poster's interests or need.