They need to get the battery life above "1 day". I think that's what's stopping MANY people. In my case, I almost need sleep tracking (diagnosed with a sleep disorder).
I think this is a great future market. It's quite unfortunate that, although the Apple watch has a pulse oximeter in it, the functionality is currently disabled. Reportedly because it would then be a medical device in the eyes of the FDA.
Good sleep tracking, just that one function, could conceivably sell millions of units.