From what I read, that feature is currently not there, it may come in 'New forum'. You may want to wait a few months more. Just click on the threads you want to read and then click 'mark all messages as read' button(top-right). It will help you a little(till a new post is made). It is annoying for me too but I think we will have to forget some features.
OR you can try this:
If you click on watchlist, you should see threads that have new replies to them that you have not looked at yet. When you click on your watchlist and see something you no longer want to see copy the thread name, click on edit, then use the "find" feature on your browser to find the thread and delete it.
~~MZ~~