Except it delivered none of the "wow!" that Gmail did. It was slow, didn't really seem to have an immediate purpose (where as when Gmail came out, most people I knew had hit some sorta inbox-size limit before and knew that pain point)
and so on.
That coupled with the fact you could only communicate with other Wave users made wide-spread adoption impossible.
Facebook users can only communicate with other Facebook users, but wide-spread adoption happened.
Maybe the idea for wave to replace email was flawed, but it works well for specific documents and folders like in Google drive.
It was like Tweeting hashtags combined with Google Docs. It was very immediate, and you'd see people showing up to discuss in realtime.