Most thermostat wiring is going to be a two wire switch, r to w, and most programmable electronic stats run with AA batteries. Very rarely will the C, common, of the stat circuit be run to the stat location.
I did some troubleshooting of a Nest stat, homeowner install, gadget freak early adopter. He had tried to troubleshoot it and was telling me I had to run the C wire to all the stats, per the factory support.
I believe the Nest self powers from a small parasitic current draw between the R and W wires. Nest says the stat is rated for two wire, R to W, installs, but says if you have trouble, to run the C wire to the stat. I tried to get the operating characteristics from the factory, to see if the system was suitable and working or not, but they like to treat it as a black box, no internal details.
It has to be charging an internal battery or extra large capacitor to run the processor and wifi. It must be self powering from the R to W in series with the heat relay coil and remaining idle most of the time with bursts of processor and wifi to minimize power consumption.
He had a bad stat that was really flaky, but the factory diagnosed it as a power issue and recommended running the C to the stat. I beat on them and said no, your rating says the stat will work on the typical two wire, R to W, application. Once it was diagnosed as a bad stat, as far as I know the factory swapped the unit. It was a new install.
My only concern would be the longevity of the internal rechargeable battery. Usually batteries are toast after five years, and if that causes the Nest to fail, I would say they have a design flaw. Thermostats are critical equipment. They would need to have an expected lifetime more than 25 years to be saleable. The factory would not give any information on this when I asked. Their fallback position was to say, run the C wire to the thermostat for power issues, which is not practical for old work installs.
Other than that, the wifi internet access, software, and learning program are features some people will use. My impression is their marketing is a little ahead of the product. They could not answer how the nest internally stores power or how long that will last (they know power is an issue but most application will only have R and W at the stat). If they try to say Nest will save money with the learning, occupied / unoccupied temp setback, in practice most users will not make that work.
The biggest selling point would be the internet accessible remote monitoring and adjustment, for users that need that. They could be a good company in the future, but it's still early in the development of their products.