I just want acceptable performance. That's the big one for me.
Battery life could be better, and I like the idea of a display where the buttons don't look so deep, and I can see how the market might respond to having BOTH square and round versions (I confess, if it weren't so big, the Gear 3s do catch my eye). But really the only thing I absolutely must have is an OS that doesn't make me wait 15 seconds every time I do something.
"Siri, remind me to update the grocery list when I get home."
Wait.
Wait. Look at iPhone to make sure it's still only 6 inches away from the watch.
Wait.
Wait some more.
Twiddle thumbs.
Get my phone out.
Unlock it.
Launch Siri.
"Siri, remind me to update the grocery list when I get home."
<Cashier presses Credit key>
<double-tap Apple Pay button on the watch>
Wait.
Wait.
Wait longer.
Look at the cashier like it's their system's fault.
Pay and apologize to the angry mob behind me before I get stoned. Well, tomatoed.
Lines I actually heard: "Wouldn't it have been faster to use a credit card?" and "That would be cool if it worked."
Normally it's fast, but when it's not it's humiliating. Now I activate it ahead of time to avoid the embarrassment.
Finally, apps. Just about any app that has to proxy through the phone is horribly painful to use. I remember when a big industry player released an app, and I wanted to show our employees. I displayed the device on our conference room projector via AirPlay (cool!) but then couldn't get the app to do much of anything. Constant spinners. The iPhone version worked incredibly fast.
I only have about 5 third-party apps on the watch, and have tried wiping and setting it up as a new watch. Same horrible performance. Thankfully they're going to improve performance a lot in watchOS 3.