Your friends didn't need Gmail for you to be able to use it. But they did need Wave. And they do need +. There's a big difference between open protocols and closed systems.

It appears that you can interact with non-Google+-enabled accounts to a limited extent; they receive your posts as emails.

That might be the point that lets Google+ succeed: you can sign up and interact with your friends immediately, and then when they join Google+, you get to do more.

It will definitely help with the "I have an account but nobody else does" phenomenon.

I also wouldn't be surprised if its launch as an invite-only beta release was in part because of it pioneered large email storage.

Gmail concept won't work for plus as what is a social network without a network. If they have load issues they should start with influencers (like they did) and then keep letting them invite their friends. Then their friends can invite their friends and let it grow from the seed. It is wacky that my friends have to get an email update rather than experience the network with me.

Uh, what was GMail then?

When it first came out, the chosen few got invites, and they invited friends. Rinse, repeat.