From another meta question here, I guess this is a little-known feature.

Look at a question page on MO. On the right, below BLOG, HOT META POSTS, Related etc., you will find:

Click on that to add yourself to the "feed" for that question. How it works, I think, depends on how your computer has been configured. In my case, these notifications arrive in my "News".

I note that there is a "user feed" at the bottom of a user's profile page, so it seems you can sign up to be notified of whatever that user posts. Look around. You may find other feeds as well.