I don't consider myself a guru on wearables, but I can promise you that I have created and launched a smartwatch and learned quite a bit.

1. Remember, fashion is what you wear and electronics are what you carry. So, make it gorgeous or make it invisible. The first killer app is emotions - unless, you are doing it for health reasons/industry vertical applications.
2. Resist the temptation to duplicate the old UI rules of mobile into a wearable.
3. Use familiar shapes and socially acceptable practices - screaming commands into a watch may be a tad uncomfortable to many people.
4. Trying to sell your wearable as jewelry is not easy. Remember, jewelry is timeless and technology is ephemeral.
5. And finally, less is more. We are already slaves to our phones and live in a hyper-notified world. Wearables/IoT technologies should amplify our ability to absorb information and things should be done for us and not done by us. For example, lesser the engagement with your smartwatch, the better. The last thing you want is people looking at their smartwatches and driving!