Daniel Pasco, CEO of digital agency Black Pixel, writing for Medium:

Existing WatchKit apps present a UI on the Watch, while the application logic processing is actually done on the iPhone. Communication between the UI code is done via bluetooth between the Watch and the iPhone, which adds considerable network overhead and latency, ultimately resulting in a fairly slow interaction and experience.

watchOS 2 applications, on the other hand, collocate the logic execution and the UI together in the Watch itself. This makes the application experience much snappier, reducing the amount of time needed to interact with your (indispensable to the user) app so that they can get back to more urgent things such as avoiding oncoming traffic, talking to their spouse, etc.

watchOS 2 brings a major evolutionary change to the Apple Watch experience. You’ll be able to do much more on your watch without necessarily having access to your iPhone. This is a major opportunity for developers and a real step forward in potential experience for the Apple Watch user.

watchOS 2 was announced at WWDC and is available to developers now.