Why email is a problem for Google
You'll notice that Google has made repeated attempts to replace "dumb pipe" Gmail with something smarter. They tried Google Wave. That didn't work out.
They hoped people would use Google+ as a replacement for email. That didn't work, either.
They added prioritization. Then they added tabs, separating important messages from less important ones via separate containers labeled by default "Primary," "Promotions," "Social Messages," "Updates" and "Forums." That was vaguely popular with some users and ignored by others. Plus, it was a weak form of mediation -- merely reshuffling what's already there, but not inviting a fundamentally different way to use email.
This week, Google introduced an invitation-only service called Inbox. Another attempt by the company to mediate your dumb email pipe, Inbox is an alternative interface to your Gmail account, rather than something that requires starting over with a new account.