With email, there already existed norms which could be taken from real life and applied to the new electronic analogue. So the new technology dropped pretty neatly into an existing cultural structure - which then evolved.
But Google Wave knows no such existing cultural structure I can think of - perhaps there are certain etiquette rules that apply to multiple people talking in a group at a party while simultaneously writing on a single piece of paper. But if there are, I don't know them.
So adopters of Google Wave will face social challenges as well as technological ones, and I don't think Google has really thought this through as well as they've structured the protocols and technical infrastructure. Start to consider Google Wave being used in a corporate environment, and you've got legal and commercial challenges on top of that - I shudder to think how a contractual discussion could be conducted over Google Wave. You might answer that it won't be, we'll still rely on email, but if we do, then why add another communication channel?
We shall see - though it speaks of how deeply Wave challenges current paradigms of online communication that no-one - least of all me - is prepared to offer concrete predictions regarding how it will be used.