I think releasing Wave as its own product, via private beta, was a mistake. If they wanted to reinvent email, they should have done it with Gmail, similar to what Apple did with iMessage.

Gmail could operate as it always has, being an email client that integrates with every other email client. But when a Gmail user emails another Gmail user (or a group of them), have the thread automatically turn into a Wave, with everything that entails. This gets the new features where people already are, creates an incentive for people to move their email to Gmail, and doesn’t create yet another inbox for a person to manage.

I always saw Wave as a business failure, not an engineering one, for this reason.