I think WatchOS 3 and Apple Watch Series 2 are a very simple story. Apple Watch had clear strengths but equally clear weaknesses. Apple identified what was flawed and went back to the drawing board. They identified what people liked best — health and fitness tracking — and made them even better.
John Gruber nailed it.
I updated to watch OS3 and I feel like I have a brand new Apple Watch. Funny note, when I bought the Apple Watch Sport (series 1 now) for my wife she asked if it had GPS and if she could swim with it. Those were her first two questions....with Apple Watch Series 2, now I can say yes. Nice.