I don't think the lack of "federated login" is unintuitive. You wouldn't expect going to gmail.com and logging in with your Yahoo credentials to work, right?

Having a "federated login" service would probably either add a ton of complexity for instance owners, or someone would implement some super naive and insecure centralized solution, leading to a bunch of people's creds getting stolen.

Getting the post/<id> thing to work across instances would be a pain too, because it would require instances to all coordinate post IDs to ensure collisions don't happen, since far as I can tell, the id in the URL isn't globally unique.