There is a limited "A look inside" view in each Mastodon instance's web interface, but it doesn't compare to reading a subreddit anonymously where you can basically read everything without joining.

More generally, my question is how do you meet people who are talking about something interesting to you? (Considering that there aren't many people using Mastodon yet, I don't expect to chat with people I know.)

The web forum/subreddit model makes it considerably easier to find interesting subject-based communities. This doesn't seem to be what Mastodon was designed for, but it seems like a good idea.