As I understand it, the nest does have a humidity sensor and activity sensors (I guess since they are part of the one device there are limits on how capable they are). It also figures out how fast the building changes temperature and accesses the outside temp from the internet (this is weak, the temperature at an airport 50 miles away can be dramatically different).
I still wouldn't buy one as I don't quite see the value proposition (and am not deeply offended by mashing buttons to program a schedule), but it seems like a bigger step in the direction you are talking about than you think (and I guess it helps to have smart thermostats in the wild when you start trying to sell smart dampers into homes).
The nest seems like a poor compromise. It's easy to wire up but pricey and the benefits seem questionable given the price. As the Animats points out one option would be to add a lot of home automation given the amount of processing power the thing has and save a significant amount of energy.
However, a vary dumb thermostat with a humidity sensor could save you a fair amount of energy in humid areas over a simple temperature sensor. After all a humid 80 and a dry 80 are different, not just in feel but also in how fast mold grows. Basically, set a max temperature for the humidity level, and decide if you can just vent outside air, or if you need to dehumidify incoming air, or even if you need to heat/cool the air. The other option of simply running a dehumidifier uses a lot of energy and tends to create an overly dry environment.
I still wouldn't buy one as I don't quite see the value proposition (and am not deeply offended by mashing buttons to program a schedule), but it seems like a bigger step in the direction you are talking about than you think (and I guess it helps to have smart thermostats in the wild when you start trying to sell smart dampers into homes).