Hi, thanks for reaching out.
Maybe I misspoke. Maybe I meant "tree view" and not "thread".
I was aware of the reply feature, and yes, if you read a reply and you click on the original title, you get to see all messages attached to this original message, in a window to the right, that you can navigate (more or less).
But AFAIK you can't reply to a reply (if you do, it's a reply to the original message); it's probably a feature (back in the days, the Joel on Software forums worked like that, and Joel Spolsky was actively defending this option in various posts).
But it makes it difficult or impossible to actively monitor different threads or topics. Threads are not collapsible. The UI actively favors the more recent messages, you have to actually click on "load more messages" to see earlier messages (and if you change channels and come back, the earlier messages are gone, you have to click again to see them).
Anyway, I don't mean to be negative on the product which is very well polished and works perfectly; it's just that the choices that it makes seem ill suited for a very international and asynchronous crowd.