I can't help but think that Google Wave missed out. They were no where close to being ready when they released it. Some of the no-brainer features like deleting waves and removing someone from a wave is just being introduced in the last few months. The speed was always an issue, but it only improved in the last few months. And the UI, while beautiful, is not intuitive (three panels as opposed to two panels in gmail?).

The whole concept is great, but their delivery failed. I am sure there always will be niche users who love wave and use it often but doubt it will ever have anywhere close to wide adoption like email.