A post above outlined that there are three types of communication: synchronous (two-way), asynchronous (one-way) and broadcasted. Google Wave is a combination of the three, in that you can utilize all of them at the same time within one system (except for email, which can interact with other systems as usual).
Because it is restricted to one system, namely Google's, it won't reach critical mass until either other current systems can work with it somehow or Wavelets become an open standard and can be added onto current systems as-is. I hope that Google's pushing for the latter direction, as it's quite an interesting technology...
Yes, both four and five finger pinches would use your thumb.
But try it - it's really awkward to hold your pinky (or any other single finger) up for any length of time (at least I find it so). Perhaps it's not for some people, the way some people can easily do the vulcan thing and others not (I can't at all).
If you are going to totally redefine user input, you have to take into account something a large number of people cannot do.