Although the "by invite only" strategy worked for gmail, I think this is what has ultimately killed (or at least maimed) Wave. Too many people tried it out, were pretty excited, but then realized they couldn't talk to anyone they knew, and getting an invite to the people they knew was only by the unpredictable grace of Google. This time the invite-only strategy was a buzzkill. This is probably a case of Catch 22 though because if they had launched it with open access to all it probably would have been slashdotted.
At this point, opening it up to everyone is probably a sign that most people have given up on it and Google recognizes that it won't cause much of a murmur on the servers since few will sign-up. One of the real sticking points for me is that you cannot permanently delete a wave. Sure it goes in the trash, but there is no way to empty the trash. Google should at least give us the illusion of privacy by at least making it appear like we can delete content tied to us!