I don't think they will tell us. I think they will say nothing, and just let it be found out, by consumers after buying the product.
Unlikely. They will eventually give out the tech specs they have included in the watch so people can make a decision about buying it. There are only so many independently wealthy fanboys out there who will blindly throw cash at a product so they can find out what it does. We're not all Nancy Pelosi's.
If they want to draw customer away from other products in the same market space they will need to show how they are better than product X and not wait for word of mouth. At $350 it's not a cheap throw away you can take a chance on.
I'd like to think there are hide sensors in there for pulse Ox, and such that will get turned on via software updates when the kinks of interpreting the data accurately are worked out. Hydration sensor wouldn't be bad either.
I think gen 1 will be HR to start with and Apple needs to do a better job with presenting the wearer with accurate date than its competitors have done to date. The current crop of optical HR monitoring devices suck.