You want threads not because you want to talk to a specific person, but because you on to talk on a specific sub-topic. Usually a post contains not one thought, but a lot of them, so people who comment, do comment on various moments. As discussion evolves, those moments tend to either separate in different threads, if there’s an option, or discussion will stop because it will feel like talking in a loud noise, in a room where a lot of people interrupt each other.
When you have a real-life meeting with a group of people on a broad topic, it usually looks like “threads” too. Someone picks the first particular issue, then some of people in the room discuss it, some remain silent. When the first issue seems to be done, someone else brings up the next issue, and it’s discussed by others, and again it would be not all of the people in the room usually. When this order of talking about just one particular moment is violated, people usually have stress and headaches after the meeting, because they are forced to move to the next question in the middle of the previous one, and hold they thoughts on the different questions simultaneously.
I do use livejournal just because it supports discussions, where you can talk to people on some thoughts you have after reading, and you can do that in a separate thread, where only people that are interested in talking about this particular thing will post. When the comments are flat, all you can do is just throw your thoughts to the author with no hope for the response, especially in a situation like this, where a lot of people will read and comment. For example, even if someone would want to reply to me to this particular comment, I wouldn’t probably see that, because I wouldn’t read all the comments to this entry, cause half of them will be just thanks to the author of the post, or extended “+1” comments, and others will go on another thoughts raised by your post.
And in the livejournal communities there are a lot of productive talks and discussions in comments, because a good post in a popular blog or a community raises like 10-20 of them. People comment to the post, and then some people comment to their comments, because they have something to say about what was brought up in those comments. Result feels like a communication, not just post and thanks, or question and answers, which is just one round of what we normally do when we communicate.