Key idea: if N people sending emails to each other are all on Gmail, then Google can keep only one copy of the email discussion and show it to all N of them.
This is why gmail-to-gmail emails arrive so quickly.
More generally, rather than using point-to-point protocols to make N(N-1)/2 pairwise connections between N people, there are certain new possibilities if everyone just maintains a single link to a document (a Wave) hosted on the server.
Wave is kind of the inverse of P2P. Downloading is more efficient if you can simultaneously download from N people. Uploading is more efficient if you only need to upload to one server.
This is why gmail-to-gmail emails arrive so quickly.
More generally, rather than using point-to-point protocols to make N(N-1)/2 pairwise connections between N people, there are certain new possibilities if everyone just maintains a single link to a document (a Wave) hosted on the server.
Wave is kind of the inverse of P2P. Downloading is more efficient if you can simultaneously download from N people. Uploading is more efficient if you only need to upload to one server.