Seeing the intrinsics APIs get filled out- in the open, no less- has been pretty exciting. The fact that something like AES would be implemented competitively in C# is not something I would have predicted even five years ago.

It's remarkable how fast the language and runtime have evolved for performance. It wasn't that long ago that I was manually inlining Vector3 operators to try to get a few extra cycles out of XNA on the Xbox360.