I actually found this very hard to find all in one place, so I put it together myself at the Apple StackExchange, where it can be updated and maintained. It might be useful to some:
apple.stackexchange.com
Happy to hear any corrections, too!

What are the major differences between Apple Watch versions?
With all the different versions of the Apple Watch, what are the major differences between them? Apple no longer lists tech differences between hardware they don't sell, so it's difficult to find the
Happy to hear any corrections, too!