So far, on the Apple Watch, I'm loving the Time Travel feature. With a quick turn of the digital crown, I can see my calendar events, the weather, and other dynamic elements on my watch face change to match whatever time I am scrolling to. It's a super fast and super convenient way to take a quick look at the day ahead.
Spotlight search now shows many more results, including from the Web, Wikipedia, weather forecasts, as well as responding to natural language search terms, such as "Emails from Harold". A system-wide (and beyond) search means less time spent thinking of where would be the best place to search for something: in your email folders, on the web, in documents you've already created or downloaded, or so forth.
watchOS 2 is now available for updating via your iPhone's Apple Watch app. El Capitan is presently only in beta form, but will "soon" be available on the Mac App Store (likely in the next few weeks at most).