I don't know if it can really be whittled down to one thing, but I'm really just throwing out somewhat educated/informed guesses on this. I do remember a study that showed economic signs basically become meaningless after a set period of time. GDP used to be a fairly good way to look at an economy, but that the stock market has basically fucked that all of. GDP is supposed to be a sign of productivity, but that is hardly the case anymore, with our stock market. Production itself is very important, but it depends upon what you're producing as much as if you are producing.
It's probably a combination of things, unemployment, production, distribution of wealth, health of the citizens, state of the infrastructure, etc.