In particular, it lacks functionality to turn itself off at night, to set turn on/off temperatures, etc.
Plus, I wanted one that would accomodate my laziness and back off (let house get warmer or colder) a bit when I'm not around. And then I thought: a good heuristic if I'm around is if I'm using my computer.
And the interface to the heat/AC system is trivial: a couple wires you cross and it goes.
So, I set out to start researching thermostats and found that somebody has already done this (of course) except they took it ten levels above where I'd go:
How absolutely hard-core is that?
Now I want to build my own more than ever. :) But *sigh* ... I have enough projects. Maybe I'll just cave and buy one at a store.
P.S. This is also rad.