>May be, it will interessing to put the treeview on the side left like Thunderbird Newsgroup with the font bold for the new message.
>And on the middle you have the same interface but with only thread filter by left side.

We cannot change the core design of the site. This has been used for 17 years and it would be a very bad decision to change it.

Your interest seems to be targeting at obtaining a similar view as what you have elsewhere. If this is the case, which seems to be targeting at obtaining one message under the other, then the View the entire thread option would be an option. This is an option which appears on the message toolbar. If you click on it, you get a report similar to that

But, the overall way of presenting the threads are in a threaded envrionment, and only what's new. The engine you are talking about, for most of them, if I understand correctly, are showing all categories all the time with a number of the total message in them with a link to get them all. This would be as showing all your voice mail messages on your phone everytime you listen to them even if you would have already listened to them before. One of the focus of this engine is to offer something more poweful in the way we present the messages, thus threaded, and only what's new as this is what we want whenever we log in or refresh to get new messages. The engine however offers a bunch of tools to go back in time or to gain access to very important messages, such as replies, with a one click access from the Reply option on the top toolbar.