For example this page shows the instance's timeline without logging in: https://mastodon.sdf.org/about
If you like someone's toots there, you can follow and message any user on SDF's instance from your account on any other instance.
As mentioned elsewhere, the only thing you would gain by creating an account on sdf.org is to have your toots appear by default in the local timeline of other sdf.org users.
Although it might be better to say that that's the manifestation of the real problem. The real problem is that I want to be able to talk about books sometimes, even though I'm a musician so I registered on the musician instance. If there's a way for me to have conversations about books, I don't care whether it's an "instance" or not. I just don't want to manage two separate accounts. (As an end-user, none of my end-goals mention the word "instance".)
For a federated design I'd assume the main difference among instances to be code of conduct/ToS.
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.
alice register on instance "music"
alice wants to see what's up on instance "books"
Is the problem that the default behavior of Mastodon requires alice to register a new account on "books" just to see that instance's timeline?