Joining a Discussion
I have noticed that many new comments in any discussion just repeat what has already been said. This is probably because when you open the discussion on a lesson the first thing you see is a box to type into. So you get many repeated comments some of which have already been answered if only you had read down the page. I think DL should put a sentence at the top saying "please read what has already been discussed and you can add your comment at the bottom" and put the typing box at the end.
Did you try clicking on the text box for a sentence discussion? If you do, you will see a message about reading comments before posting. This message is quite recent so either there are already many old messages or people still miss the warning.
There are two problems with that notice, first it doesn't appear when clicking the sentences in the forum. Second, it doesn't appear when you reply to someone even if you're within a lesson. So a thread may be huge because people keep replying and respond to messages using the forum rather than from a lesson.
I think there is an even simpler solution- put the typing box underneath the comments. That way, you have to look at the other comments before you can post yours.