Auto-creating an account is a powerful possibility, but it is also *unexpected*. We need to take into account two things: the user is used to register, and this is foremost a psychological act that exposes their intention to join a system, so we cannot simply bypass/impose this step; it could be confusing or irritating.
Second, the management and backup of the private key need to be explained asap to avoid lost or duplicated accounts.
I think the solution is minimal and frictionless registration, with a simple but solid explanation of Nostr's innovation, combined with a persistent warning that is only hidden when the user has secured the account.