I believe you have to follow a "Tag" first. In the new lingo, "Tag" = "Forum" and "Discussion" = "Thread". There are "subtags" too like "Automotive" under the "More" Tag. To follow a Tag or Subtag go to the main page for that Tag (sub) and click the "Follow" button.

To follow a Discussion in a Discussion view click the three vertical dots on the right and choose Follow. You automatically follow discussions that you create or post.
