You're right, I think, in complaining that this is too verbose. Yudkowsky has some very good writings, but this isn't among them. The key point, I think, is here:
"'I can’t create my own reality in the lab, so I must not understand it yet. But occasionally I believe strongly that something is going to happen, and then something else happens instead. I need a name for whatever-it-is that determines my experimental results, so I call it ‘reality’. This ‘reality’ is somehow separate from even my very best hypotheses. Even when I have a simple hypothesis, strongly supported by all the evidence I know, sometimes I’m still surprised. So I need different names for the thingies that determine my predictions and the thingy that determines my experimental results. I call the former thingies ‘belief’, and the latter thingy ‘reality’."
I don't think the essay as a whole communicates much more than that point, but it's not a bad one.
This is an interesting point, though it seems to entirely correct to me - even if I had complete understanding of all physics laws and could predict results of any experiments I can imagine - they would not mean the reality does not exist separately from me. If I could cause any experiment produce any result - including same experiments producing different results in the same circumstances - then I would have a good claim on controlling the "reality". How to determine if I caused the result and not just predicted it is an interesting topic, which probably requires some careful design, but since we know about existence of no-deterministic processes, this should not be too hard to do I suppose.
In other words, what is described here is a sufficient, but not necessary condition for existence of the reality. Though thinking more about it, it's not sufficient either. We know people that are not able to predict or control their own thoughts and actions. They are usually considered "mentally ill", but that's beside the point - it is so only because such mode of existence is very inconvenient for the person and hinders his interactions with the society. But if there was not reality except for the said person's mind, this person still could make wrong predictions about what is going to happen. If fact, one does not have to be particularly crazy for it - many people, just this season, claim with complete belief and certainty, that they will do certain things very soon over which they have full and complete control - such as start exercising, eat healthily, cease smoking, improve their behavior in some other way, etc. Many of them will discover, some time later, that it did not happen. Does it mean something other than their own will made it so? No, I think it does not - it just means they were fallible.
So it looks like the experimenter being wrong is not either necessary not sufficient condition for the existence of the reality independent from the said experimenter. (Note of course I don't claim it does not exist - I just say the proof offered in this paragraph does not prove what it intends to prove).
I think he also paraphrased a quote I like fairly well. I don't feel like reading that whole paid-by-the-word essay again, but it goes along the lines of "reality is the thing that exists whether or not we believe in it." A reasonable analogy would be "truth is the thing that is accurate regardless of our opinion about it." I am obviously unfairly dismissing relativism outside of a particular range of variation since to do otherwise makes most discussions about anything pointless.
"'I can’t create my own reality in the lab, so I must not understand it yet. But occasionally I believe strongly that something is going to happen, and then something else happens instead. I need a name for whatever-it-is that determines my experimental results, so I call it ‘reality’. This ‘reality’ is somehow separate from even my very best hypotheses. Even when I have a simple hypothesis, strongly supported by all the evidence I know, sometimes I’m still surprised. So I need different names for the thingies that determine my predictions and the thingy that determines my experimental results. I call the former thingies ‘belief’, and the latter thingy ‘reality’."
I don't think the essay as a whole communicates much more than that point, but it's not a bad one.