1. It’s mostly about simplicity
Apple’s guidelines emphasise the device’s personal, holistic, lightweight nature. It’s something “attuned to the wearer’s presence", “[blurring] the boundaries between physical object and software”, with apps “designed for quick, lightweight interactions”. Don’t expect Bioshock on your wrist, basically.
2. Your iPhone is the engine
An iPhone is, unsurprisingly, mandatory, because it does the heavy lifting. For now, the Apple Watch is an expensive wrist-based notification centre of sorts, or is used for performing basic interactions. An Apple Watch app, says Apple, “compliments your iOS app; it does not replace it”.
3. There are two screen resolutions
We knew Apple Watch was coming in two sizes, but the dev literature confirms each has a unique display resolution: the 38mm model has 272 x 340 pixels; the larger 42mm one has 312 x 390. Apple pundit John Gruber reckons both will match the iPhone 5s and iPhone 6 for sharpness (326ppi); Apple adds there is “no need to create non Retina images”, so there’s no cheapo ‘chunky pixel’ Apple Watch on the way.
4. Apps offer three ways to interact
Information can be presented by manually launching a Watch app from the Home screen, for an “in-depth user experience”; this is where most dev efforts should be focussed. Otherwise, there are optional Glances and Notifications.