[Laremere]

From what I've heard: Google docs had multi-user editing, but it wasn't real time. Periodically the document would upload, be synchronized with other copies, and then update on the clients. Wave's real time cursor, synchronization, and conflict resolution algorithms were integrated into docs.

[snewman]

You're correct regarding the original Docs implementation. I had thought that the rewrite (launched around 2010 or possibly 2011) was primarily based on the architecture of Sheets. But it's possible there was some Wave heritage I'm unaware of.

[QuercusMax]

Additionally, I believe the commenting and suggestion features were added to Docs after Wave was turned down; my recollection is that that bit is one of the main things that changed about Docs after Wave.