I've had to help a client with something not exactly like, but with similar properties as, Google Docs. One of the big properties they had to engineer in was "the doc should eventually look the same for all open browser tabs on the same computer".

The other big question: what happens if user A makes change X and user B makes change Y? There's a lot of outcomes the product can pick between, but whatever they pick it should be consistent. That consistency in conflict resolution is a good property to model.