Here's what I don't get -- why have the display to begin with? Why can't the whole thing be programmable via API/Web/Mobile App/Watch/Whatever.
Ideally that's what I'd like -- some kind of intelligent device that lives inside of my house that connects to my boiler and AC unit, as well as other major appliances if possible and provides an open API for people to build on top of, and can connect to my router.
If someone wants to make a thermostat that looks like Nest, then, it could be something that you just stick to the wall but requires no direct wiring. But it also means that I never need to buy a new thermostat: if I get tired of the Nest, I can switch it like I can any other app. Or just keep it controlled from my phone or tablet or the web.