So consequently figuring out what makes sense on a Watch screen is going to be the real hurdle to overcome. All kinds of information could be handy on a small screen. A lot of people decry the Watch by saying "why not just save your money and pull out your phone?" but I think one day we'll be saying "why pull out your phone when you could just look down at your watch?"
My company's app has a lot of info for our employees and customers. But there was some info our CFO wanted to know on a fairly regular basis and he didn't want to log into some web site to see it. Or some app. He would instead pester someone to run a SQL query for him. So I added a Watch complication to our app. It puts these numbers on the screen for him. He can use Time Travel with the digital crown to go back throughout the day. He can drill down to the actual app and refresh on demand to his hearts content (Complications are only refreshed on a particular budget). He loves it. And the people who used to have to be pestered for it love me for putting it on his watch screen.
That's the sort of thing the Watch is good for. Quick pieces of info on demand or refreshing in the background. Instances where it would be easier to glance at your wrist than pull out your phone, unlock it, open an app, etc. Instances where you don't want to walk around with your phone in your hand where it can be bumped out and dropped or stolen. I use the Wallet app on the phone and the 7-11 app to scan my 7-Rewards card barcode and everyone just thinks it's the coolest thing ever even though I feel like a dork doing it. And then it feels incredibly primitive to dig out my wallet to pay when I could just use the watch with Apple Pay except 7-11 doesn't take it.
I think some day when the right apps are out for it it'll be as vital as our phones.