Jeff Gamet, MacObserver:
If you’re planning on using the new heart rate monitoring features in watchOS 4 on your original Apple Watch think again because they aren’t there. Apple Watch Series 0, as it’s now called, can track your heart rate, but the new monitoring options require an Apple Watch Series 1 or newer.
Solid point, good to know.
Side note: Here’s how to tell which Apple Watch series/model you have.