"Denormalization might be important"

Denormalization is still important. Taking that comment example a bit further, if you need user records tied to the comments, you're going to want to include all the user information that you need for displaying the comment in the comment record itself. Mongodb doesn't have joins. So if you have 200 comments on a post by different people, you're going to have to grab those users from mongo in a separate call, and if you're sloppy about it, 200 separate calls.

If your users really need to change their user names, you might run a job that updates the comments async.