Sometimes when I view a thread, my kneejerk reaction is to hit the reply button and move to the next thread. Granted, it is impractical to read all the replies before you make your own. But I think it pays to give some consideration to the post preceding yours in order to keep the conversation on track. This without losing focus of the original thread.
Well, I generally don't delve into deep conversations. But, when I do, I read everything, but I make sure the OPs questions are answered. Of course, when some thing creates an emotional response in me, I do what you do too. But, rarely.
Sometimes I skip the other replies and sometimes I tend to read the whole thread, it depends on how interesting the conversation is and how much time I have. But I always make it a point to stay on topic.
I usually skim the whole thread to see what other people have put because I don't want to repeat what someone else had said. I read in detail every reply to a thread I have started because it's something that is of interest to me. It's important to stay on topic and I know how easy it can be to go off on a tangent.
I like to read the first post with all of my attention as it will give me all the details on what I have to write about. I do read the replies sometimes because I get curious about others experiences or if it's about a topic I was looking information for. I usually get inspired by the initial post and write my reply before I forget what I wanted to write on point.
How many of the replies I read depends on the number of posts a thread has. If the thread has too many replies, I don't read the older ones. However, the content of the original poster is must-read to understand the nature of the conversation. Apart from that, I read couple of most recent conversations.