extension without the sandbox is the biggest win for all - more 3rd apps to use iOS
Swift...will see if its really that easy to use...
Well with the industry command that Apple has, you can be sure it's not going to be whether it will be adopted. It will be adopted because just the integration with Metal is good enough for all developers to jump in. So if you are an iOS developer, don't even need 2nd thoughts if you need to learn this new language. All the new functional concepts is going to make some part of it header than Obj-C, but that's already how the leading programming languages are heading. Functional is the future of programming languages.
As for extension, it is definitely exciting to see Apple going liberal, but it is still within sandbox with controlled interfaces via the core. That will require explicit collaboration. Meaning software need to be written in mind for plugins, and other developers can participate. So most likely we are looking at mature applications using this technique to entice developers to contribute into their application.
One good use is the means to create SSO among a group of applications. The sandbox model doesn't make it easy and possible. You might also see Microsoft Office using this technique to create more collaboration between their own suite of applications.
Definitely we are looking at Swift reaching Mac OS X. Apple wouldn't spend the effort to create a brand new language so that it's only available for iOS. Mac OS X will get it in time and with Metal also eventually onboard, we are looking at a new platform with much of the OpenGL stripped away. That means possibly Parallels using it to create much much faster 3D acceleration within guest machines.
The new features sets in both Mac OS X and iOS are definitely exciting, but in my opinion, what makes the time for me to sacrifice my sleep worthy in this WWDC is the development framework portion. It is really mind blowing, I'm impressed with the boldness of Apple to make such a move. It's somewhat a game changer.