1. Battery life. You can't run a Marathon with an Apple Watch, nor can you wear it all day and then use it for sleep tracking. I need 3 days of charge during normal use, at least on par with my Fitbit Surge.
2. Poor biometrics. Heart rate monitoring is very infrequent. Again, I expect HR to match my Fitbit Surge. Likewise, I'd really want to see some additional metrics... blood oxygenation, at the very least.
3. Untethered usage. GPS and cellular service without having my iPhone on me would be huge. This is a lot less important to me than the two above points, though... especially since battery life would likely suffer, and battery life is the one thing Apple Watch needs most.
4. Charge speed. It takes over 2 hours to fully charge an Apple Watch. Given the poor battery life, that's nuts. This needs to be cut down to 30 minutes for a full charge somehow.
5. Water proof. There is no excuse for the Apple Watch not to be fully water proof. I don't mean water resistant either, but "I can swim laps in the pool while tracking my biometrics" water proof.
That's it for me. My guess is won't see all these improvements until Apple Watch 4. Until then, I'm sticking with Fitbit and Garmin wearables.