I am not going to even bother talking about what is in it, I am reading it like everyone else! But go get it, install it into your favorite virtual machine development environment and see for yourself.
I like what I am reading and if I get a chance I will talk about it more later. :) But I did like reading this on ScottHa's Blog:
On the .NET 3.5 side of things, since this is an SP (Service Pack), yes, some stuff goes in your GAC and gets changed. However, the changes are completely additive. That means if an API's method signature has changed, that's a bug that we need to fix. It will be a fully compatible service pack release. It shouldn't break any of your existing code.