I'll already be very happy with a device that at a glance, tells me what time it is and when my next appointment is. I would strongly consider buying one just for this alone. In meetings, it comes over far ruder to grab your phone and start tapping away than to quietly glance at a watch.
The fitness tracking features will replace the device I currently use. Big win.
If I can control my podcast and audio book playback without getting my phone out of my pocket, then it will be a win when walking or biking.
I do enough travel that wrist-based navigation while walking through an unfamiliar city is attractive.
And basic home automation like adjusting my wemo lights or thermostat are a big win if it means I don't actually have to have my phone with me while shlepping around in the house.
A lot of people seem excited by notifications. I will likely turn most notifications off. I'm bombarded by enough distractions through the day already.