I remember when Google Wave was hyped up to the point where people were begging others for a beta invite. When I first got access, it seemed pretty interesting but oddly enough, nothing really motivated me to use it again.
That whole "begging for an invite" thing was, in my world, the reason we couldn't use Wave. When you have a project with 10 people, and 3 of them can't get on Wave... it's game over. Back to email/IM/whatever.
I never understood why, beta or not, anyone would release a collaboration tool without the ability to invite collaborators.
I mentioned this elsewhere, but invites were queued. You had 10 invites and you could send them all out the first day you got into Wave. But it might have been days or weeks later before their received their invitation. Mine came 2 weeks after a friend invited me. My sister's arrived a week or so after I invited her. This killed the momentum, it took too long to build up the social network which this system depended upon.