Well, you can also just watch the derivative? When's the last time the OS had a major feature that was aimed at developers? I remember the early OS X releases coming out with advance after advance. The rumored ZFS integration was the peak of this awesomeness... At least dtrace was integrated.
The last thing I've seen is transparently compressed files; not exactly exciting. I think we hit the peak around the leopard-snow leopard range.
XCode 4.x. Still crappy, but way better than Xcode 3.x.
LLVM.
Continuous improvements to the Obj-C 2.0 runtime like tagged pointers and Automatic Reference Counting.
Versions, iCloud, and associated APIs for document persistence.
MacRuby framework.
The last thing I've seen is transparently compressed files; not exactly exciting. I think we hit the peak around the leopard-snow leopard range.