The invite only strategy of Google Wave seems like a complete mis-step to me. It makes absolutely no sense unless people you know are also using Google Wave.
Sure but you only get one chance to impress. How many people will be bothered about trying Google wave again when they think it is ready? Still a mis-step IMHO.
Limiting access gives Google a good chance of introducing a finished product. Something that will impress. I very much doubt that Google would impress all that many people with Google Wave as it is now. Me, sure, but not my dad.
Also: invites are not that hard to get to in this day and age. So you’ll be able to at least invite a few friends.
GMail Beta started out invite-only, and was that way for quite a while, thought by the time I got into it, I didn't have any friends who needed an invite.