Wave was very simply modern email. Conceptually, there's nothing more to it. It was a new protocol that effectively filled the same basic needs as email, but left room for first class support for the types of features we've either hacked badly on top of email or still lack today - proper threading, traceable message amendment, rich messaging & multimedia support that's not a MIME hack & an improved security posture.
I think that was maybe a secondary problem with Wave: it was marketed as some new voodoo that ended up smelling like a hammer seeking a nail, when it was really just Email 2.0