5. They have to sport functionality that is actually worthwhile in such a form factor.
Most of the head-scratching about what smartwatches could do revolves around how the size of the display and the general level of minimalism for the whole device constrains the kinds of interactions you can have with it. We're used to what can be done with a smartphone display a few inches across, but a smartwatch's smaller and inherently less functional display is going to be a designer's — and engineer's — challenge to make useful. Good luck.