The error is this: the reasoning assumes that humans desires are designed in a way that makes sense with respect to the way reality is. In other words, that we're not inherently deluded or mislead by our basic nature in some (subjectively) unacceptable way.

Interestingly, this is the exact opposite of Zen, in which it's considered a premise that we are inherently deluded and misled by our basic nature... and in large part due to our need to label things. As in How An Algorithm Feels From Inside, Zen attempts to point out that our basic nature is delusion: we feel as though questions like "Does the tree make a sound?" and "What is the nature of objective morality?" actually have some sort of sensible meaning.

(Of course, I have to say that Eliezer's writing on the subject did a lot more for allowing me to really grasp that idea than my Zen studies ever did. OTOH, Zen provides more opportunities to feel as though the world is an undifferentiated whole, its own self with no labels needed.)

On the other hand, without a God to keep things in check, there is no limit on how horrible reality can be.

Eliezer Yudkowsky made quite a good essay on this theme - Beyond the Reach of God.

Thanks for the edit to the original comment; I was unsure whether you were arguing for a view or just describing it (though I assumed the latter based on your other comments).

Without God there's no end game, just fleeting existence.

Like the statement in the original comment (and like most arguments for religion), this one is in great need of unpacking. People invoke things like "ultimate purpose" without saying what they mean. But I think a lot of people who agreed with the above would say that life is worthless if it simply ends when the body dies. To which I say:

If a life that begins and eventually ends has no "meaning" or "purpose" (whatever those words mean), then an infinitely long one doesn't either. Zero times infinity is still zero.

(Of course I know what the everyday meanings of "meaning" and "purpose" are, but those obviously aren't the meanings religionists use them with.)

Edit: nerzhin points out that Zero times infinity is not well defined. (Cold comfort, I think, to the admittedly imaginary theist making the "finite life is worthless" argument.)

I am a math amateur; I understand limit notation and "f(x)" notation, but I failed to follow the reasoning at the MathWorld link. Does nerzhin or anyone else know someplace that spells it out more? (Right now I'm studying the Wikipedia "Limit of a function" page.)

I am reminded of The Parable of the Pawnbroker.

Edit: Original link.

Edit: this comment happens to reply to an out-of-context sentence that is not endorsed by Zachary_Kurtz. Thanks to grouchymusicologist for noticing my mistake.

Without God there's no end game, just fleeting existence.

You happen to be wrong on this one. Please read the sequences, in particular the Metaethics sequence and Joy in the Merely Real.