I've been doing a bit of programming with Spotlight on my Tiger install and it's quite a remarkable technology. It's been a while since the ease of producing impressive results made me laugh out loud with delight, but Spotlight is that kind of thing. Core Data is similar. Cocoa Bindings was that kind of technology in the Panther era, as Interface Builder was in earlier days. What delights me the most, though, is that the usability ethos that's always been part of the Mac UI also extends to the developer-level stuff, even to the API design.
This is an exciting time to be a Mac developer.