You guys brought up social sharing --
I've finally got a family member, my sister, who received a Fitbit for Christmas. I reinstalled the Fitbit app and she invited me to do a "weekend warrior" challenge right away.
Wouldn't you know it, but we stayed home, and my phone stayed plugged into my laptop all day Saturday. Since Fitbit doesn't read data collected by the Watch, it thinks I walked zero steps all day.
It really was a lazy day, but I still got in a workout around lunchtime, and none of it was read by the Fitbit app.
I can't blame Apple for this. All the data is sitting there in Health and is accessible to other apps. It's just Fitbit who's choosing to ignore Apple Watch data, lest it give people a reason to stop buying their fitness bands. I can't really blame Fitbit, then, because they're looking out for their own interests, too.
Anyway, my foray into social fitness goading may or may not work. On one hand, it's kinda interesting how I can see if my sister is keeping up her pace. On the other hand, I don't want to be broadcasting my activity into the ether every single day. I'd rather default to private, "unsocial" fitness tracking.
Back to the versatility vs complexity question: If the Apple apps were to go social, how many different services will they need to tap into? Off the top of my head, I can think of Fitbit, MapMyFitness (with maybe four apps of its own), Strava, Wahoo, Garmin, and Nike+. There are more, too, aren't there? And each sharing service can move different types of data -- Strava and Garmin can give you course maps to follow, for example, while Fitbit just shares step counts.
I see the social side being more of a hassle than it's worth. I don't mind, then, that it's not part of the stock Activity app.