One can hardly count on users always having one of the big provider mail accounts and beyond the big providers, most email is still sent across the wire unencry

One can hardly count on users always having one of the big provider mail accounts and beyond the big providers, most email is still sent across the wire unencrypted. Email isn't generally script kiddy proof, let alone NSA proof. While I like the idea of login via email link, it's only good if the login link expires very quickly. Email is not an auth mechanism, if you want to rely on the big providers to do auth, they have systems specifically for that already.