Nest is, unfortunately, an idea so dumb that it truly baffles me how can supposedly intelligent people come up with it. Any decent residential HVAC system comes with a smart thermostat already, and most of the efficiency gains are due to the efficient two-stage heat pump, variable speed blower fan, and a multitude of sensors that monitor it all and let the thermostat drive it appropriately. If I were to replace my default thermostat with a Nest, I'd actually lose lots of functionality for what: a perhaps better aesthetics and a remote access feature of dubious value.
At the very least, the Nest people should have reverse-engineered the rather trivial CAN-based protocols used by the few common smart thermostats and supported those. If you don't have money to buy a decent, modern and efficient HVAC system, you're not in Nest's market anyway. And when you do have a modern HVAC, a Nest is a step back.
It seems to me that Nest's designers only ever lived in crappy housing with old HVAC controls, and have no engineering background in HVAC. I feel a bit sad for the consumers who got tricked into buying a Nest - it's about the worst way they could have spent their money. In all seriousness, they'd get more utility had they spent the same amount on a used tablet or somesuch.