I just finished watching the Apple Watch keynote. There's one thing that repeatedly stands out throughout the course of the presentation, even though Apple doesn't explicitly highlight it.

The Apple Watch allows you to interact with rich, contextual notifications with almost no friction.

This is far more profound than it sounds upon first read. Let me explain:

TouchID has changed how I interact with my phone. Anecdotally, I've seen many members of the press, VC, and entrepreneur community echo the same. TouchID brings incredibly simple security to the masses.

But TouchID has one glaring shortcoming: it doesn't solve the security challenges associated with consuming interactive, contextual notifications without unlocking your phone. If you want anything more than a simple alert while maintaining strict security controls, TouchID falls short.

And that's exactly where Apple Watch succeeds. Apple Watch can render rich, interactive, contextual notifications on your wrist. And you can interact with them without unlocking a device. Notifications on Apple Watch are an order of magnitude more convenient than pulling out your phone, swiping a notification, and then using TouchID to actually unlock the phone.

This interaction model appears to break security by skipping the step of user verification. But it doesn't. Apple Watch can skip the user verification test via a Bluetooth LE tie to your phone. Apple Watch knows that if you're wearing Apple Watch and your phone is close by, that it's safe to render information on the watch face.

I'm not as convinced about the fitness and health value of Apple Watch given the struggle that other fitness trackers have faced in the market. However, the notifications functionality is a step function better than the status quo. Notifications will be the foundational killer app of the Apple Watch just as the multi-touch UX was the foundational killer app of the iPhone and iPad.