Yesterday, Apple released the first developer betas for iOS 11.3, macOS 10.13.4 High Sierra, and tvOS 11.3. They all contain a range of new features, being the company's mid-stream feature updates. watchOS 4.3 was notably missing, but it's finally here.
First of all, the release notes for watchOS 4.3 build 15T5165e say very little. There are no known issues listed, and there aren't any new features listed either.
Presumably, the new features will coincide with what we're seeing from iOS. For example, the new update will most certainly support Apple's new Business Chat feature, which allows users to connect directly with representatives from businesses. Users can book appointments, and even make payments via Apple Pay Cash.
Other than that, it's hard to say what's new, if anything. Just the fact that it's delayed by a day seems to imply that it's less of a priority for Apple.
If you're a registered developer with Apple, you can install the new build by heading over to Apple Developer, installing the beta profile, and updating via the Watch app on your phone.