Personally, I do like the concealed nature of threads within the main conversation, but I do not like how they are presented when opened. The whole "displayed on a narrow pane on the side" seems too cluttered to me. I'd much rather have it expand in the main conversation pane, maybe even create nested threads inside (kind of like reddit/HN).
Thread notifications are another pain point, as others have also noted. For some reason, all thread updates/notifications are clustered together in a separate tab called "Threads", regardless of what channels these threads belong to. If a thread (that I am following) belonging to a channel has an update, shouldn't that channel be highlighted? Now that I think about it, if that channel is highlighted and the user clicks on it, how is the user going to know which thread to check... all I can say is, threading in chat rooms is a non-trivial problem and while Slack gets some part of it right, there's still a ways to go.
Thread notifications are another pain point, as others have also noted. For some reason, all thread updates/notifications are clustered together in a separate tab called "Threads", regardless of what channels these threads belong to. If a thread (that I am following) belonging to a channel has an update, shouldn't that channel be highlighted? Now that I think about it, if that channel is highlighted and the user clicks on it, how is the user going to know which thread to check... all I can say is, threading in chat rooms is a non-trivial problem and while Slack gets some part of it right, there's still a ways to go.