It depends what use you want to make of your data. When it comes to technical capability for tracking sports activities, I think Apple has almost caught up with Garmin/Suunto etc for mainstream activities, especially if the rumours about this device are true. But when it comes to providing access to data and (God forbid) sharing it outside of Apple's walled garden, they're still miles behind, even with third party app support.
For me, its not really about the third party support, its a combination of software and hardware. I appreciate this is a niche case, but some of this will apply to other sports/activities which it
seems like this watch is going to be aimed at.
As things stand I can sync activities with most 3rd party services and that's been the case for a while hasn't it?
Touchscreens are great, but not when its pouring rain or snowing and I want to check something on the watch that isn't on the main screen. There's a reason Garmin and Suunto watches still have big, tactile buttons which are very analogue in feel.
These watches can also do GPS tracking for 24 hours+ which needs a big battery, but also software which can be altered so you can adjust the GPS interval to maximise battery life. My Suunto also has the intelligence to recognise that I'm doing something long and alter the GPS recording interval automatically as well as closing down other services if my battery is getting low.
Customisation of the activity screen would be good. With more fields and maybe multiple screens of data for easy swiping left/right.
And then there's the (now) basic stuff, so measuring your training load and recognising when you maybe need a rest day rather than just telling me I'm being a lazy git the day after a long/hard session. I think that's something which really needs fixed in the software as I don't think it is healthy in all circumstances to point out you're maybe having an easier day than normal. And that should be factored-in to the streaks in some way.