Subtle changes, bugs and feedback

Does this mean that having some kind of avatar is compulsory? (NB I don't think that's a bad thing, as I like avatars.)

It means that for those who have avatars enabled (to view), they should always see an avatar for someone even if they haven't set one.

It also means that those who use gravatar won't have to bother uploading one here if they don't want to... it will just detect and use their gravatar one.

Would the same effect be achieved by deleting a non-Gravatar avatar from the User CP?

Yes, that's what I was saying.