Did Wave ever really live, though? I know it had its vocal fans for whatever reason, but most people never understood what it was, even if they had heard of it and tried to use it (like myself).
The branching comments, embedded documents, and ability to create mini applets were the big selling points. But the feature were never well explained and internal implementation was a nightmare. As people could still hook in Jabber with client it was also creating issues with spamming and hacking.
They made many of the components needed to create an open source federated social networking system where separate implementations could interact with each other. So for example your user on your company's twitter/fb hybrid could have threaded conversations with other users across other organisation's fb clones (obviously with access control).
This could have been exactly the killer functionality to topple centralised proprietary Twitter and Facebook. Google should have thrown everything into it, could have turned out as impactful as hypertext.