Email was first introduced to the world over 40 years ago. Google decided to make a replacement that would be what email is, if it were invented to day. Instead, they made Google Wave.
When Google announced Wave, every tech-blog on the internet decided to simultaneously orgasm. Wave is marketed as an e-mail replacement for the modern world, seeing as how social networking sites hasn't filled that niche for absolutely everyone yet. Google next decided to make a service that requires that people actually communicate with people they know invitation-based, basically ensuring that the beta-testing period will be as far from real usage as possible.
Pictured: Every Wave beta-testers contact list
The real reason everyone got worked up about Wave is that it's from Google, which ensures that its release triggers not only excitement, but also several different theories about how this all ties into the grassy knoll.
Another reason why Wave is special is that it ties into the forthcoming Google Chrome OS, an operating system by Google designed for computers with the computing power of a pocket calculator. The common idea with Wave and Chrome OS being that everything should be stored in "The Cloud", which is basically just a fancy way of saying that a disk crash won't make you lose your essays anymore, but a terrorist attack on the Google HQ would make people all over the world fail their courses.