In our use case, most posts are intended as replies to a previous post. So it would make sense that the default main view holds single topic/discussion, and the text box at the bottom is for posting replies to that thread, and a List of available topics are shown on the side, where you also can post new topics.

This is effectively the opposite of how the UI is laid out now. The background for this request is that most users (myself included) miss to post a reply as a comment, and mess up the conversation history (even a lot of users are not even aware of the comment feature, nor the reason why we would want them to use it), so making it easier to do "the right thing" would be great.

This should of course be a preference, under system settings or some such, as not everyone would want it this way. Maybe even have it so you can toggle which way to look at the conversations on a per user/channel basis.