Well, the heart-rate monitor for starters (and I suspect, based on the optical design of it, that it was built with an eye toward measuring blood oxygen content in future incarnations). Extant activity trackers on the market don't really do it for me from a value standpoint--but I do own a Garmin cycling GPS. The Apple Watch also has enough computational oompf that they should be able to implement Suunto's Firstbeat HR algorithm for better calorie burn estimation (this is all speculative, of course).

Strava is a killer app for those who already have an iPhone: Watch Sport costs about what I would pay to upgrade to the latest Garmin Edge, plus it tracks non-exercise activity (which is something I like but am not willing to pay for by itself). On top of all that, Apple Watch comes with maps (and turn-by-turn directions) for free (something you pay for out the nose with Garmin).

Then there's the mobile payments piece. Taking out your phone is fairly low-friction, but double-tapping a button on your watch and holding it up to a terminal is even less so, plus the idea that I could go out with just my phone and watch is pretty appealing.