I've got a little theory. It seems to me that the provocative thing about the essays is their aesthetic. They're governed by a particular style. One principle in it is minimalism: compress the writing until everything extraneous is gone. Another is vividness: whatever is being said, seek the phrase or image that throws the point into the sharpest relief.
The dominant quality of the essays is that they pursue this aesthetic ruthlessly. Anything that would use a few extra words to reassure the reader is thrown out. Anything that would tone down an idea a little bit to make it more palatable is thrown out. There isn't any room for these things because the author is optimizing for something else - say, meaning per word count. In fact, an entire dimension of language, the phatic dimension, is thrown out.
So, Paul Graham's writing is radically aphatic. That's disorienting. People are used to writing that includes, among its threads, one whose purpose is to reassure you that the author is a nice guy, that he might be wrong, you can still get along even if you disagree, and so on. This is not only absent from the essays, it's deliberately excised. On top of that, what is there has been distilled for maximum impact and often touches subjects that people have strong emotions about, such as programming languages and what we're doing with our lives :). Not surprisingly, some readers feel punched in the gut. For them, an obvious explanation is ready at hand: Paul Graham's writing is like this because he is like this. He must be someone who doesn't care how others feel and wants only to magnify his own grandiose ideas.
I think this explains why people project so much emotion into what they read in those essays. "Oh... you haven't founded a company? You suck." But the essays never say anything like that. People don't read them this way because they say such things. They read them this way because they lack the kinds of things writers are expected to put in to stave off provocation. They lack these things not because the author is an asshole but because he cares about a certain style of writing. Enough, in fact, to pursue it ruthlessly... in his writing. To naively map that back to the personality of the writer is an obvious error, a reverse ad hominem. But it's an understandable one. There aren't many people who care that much about an aesthetic. (I mean "aesthetic" in a broad sense, by the way. As much a way of thinking as a cosmetic thing.)
No doubt there is a connection between an author's personality and his style, but it's hardly an isomorphism. I don't know Paul Graham, but I know he doesn't talk the way he writes. For one thing, one can point to examples (like the interview in Founders At Work). For another, nobody talks like that.