You know what's pretty cool? This potentially allows native MacOS applications to be written in JavaScript... Not that I can see any reason to do that.
While you're correct that JS can now be used to write Mac applications without an Obj-C wrapper, that's not really the point of this.
Anyway, you can already write Mac applications in JavaScript. And iOS applications! This has been true since iOS 7 and Mavericks. http://strongloop.com/strongblog/apples-ios7-native-javascri...
You can build double-clickable "applets" in AppleScript and JavaScript, but the OSA interface isn't really made for app development, just automation.
However, they do expose all Cocoa and Obj-C APIs to JavaScript, so anything is possible! They even do a demo of a standalone temperature converter at the end of the WWDC session.
I'm pretty familiar with ObjC-Javascript integration and have been working with it for a while. This change offers a nice direct system interface for automation, and one that has access to all of Cocoa.
I also didn't intend to put Javascript down at all - its more that the more complex part of building a MacOS app is working with Cocoa, and Javascript will make that more complex, rather than simpler, just because of the impedance mismatch.
I'm not sure there's a compelling use case for building such apps, but it's still pretty cool!
It actually kind of cool. And the Temperature Converter example showed a lot of the dynamic nature of the objective C runtime. This one presentation actually piqued my interest in looking at os x scripting.