That presumes we already have such a description.

We do: it's the set of all observations of human behavior. The goal of science (or rationality) is to find ever-simpler ways of explaining (describing) the data. The worst case scenario is to explain the data by simply restating it. A theory allows you describe past data without simply restating it because it gives you a generative model.

(There's probably some LW wiki entry or Eliezer_Yudkowsky post I should reference to give more background about what I'm talking about, but I think you get the idea and it's pretty uncontroversial.)

I get the idea and am familiar with it, but I dispute it. There's a whole lot of background assumptions there to take issue with. Specifically, I believe that:

1. Besides being consistent with past data, a theory must be consistent with future data as well, data that did not go into making the theory.

2. Besides merely fitting observastions, past and future, a theory must provide a mechanism, a description not merely of what will be observed, but of how the world produces those observations. It must, in short, be a response to clicking the Explain button.

3. Description length is not a useful criterion for either discovering or judging a theory. Sure, piling up epicycles is bad, but jumping from there to Kolmogorov complexity as the driver is putting the cart before the horse. (Need I add that I do not believe the Hutter Prize will drive any advance in strong AI?)

But having stated my own background assumptions, I shall address your criticisms in terms of yours, to avoid digressing to the meta-level. (I don't mind having that discussion, but I'd prefer it to be separate from the current thread. I am sure there are LW wiki entries or EY postings bearing on the matter, but I don't have an index to them etched on the inside of my forehead either.)

Here, your claim is that there's some epistemic profit from describing human behavior as a control system. I completely agree that it can be done, just like you can describe human behavior with a long enough computer program. But does this approach actually simplify the problem, or just rename it?

This approach actually simplifies the problem. (It also satisfies my requirements for a theory.)

Here, for example (applet on a web page), is a demo of a control task. I actually wanted to cite another control demo, but I can't find it online. (I am asking around.) This other program fits a control model to the human performance in that task, with only a few parameters. Running the model on the same data presented to the human operator generates a performance correlating very highly with the human performance. It can also tell the difference between different people doing the same task, and the parameters it finds change very little for the same person attempting the task across many years. Just three numbers (or however many it is, it's something like that) closely fits an individual's performance on the task, for as long as they perform it. Is that the sort of thing you are asking for?