First appeared in Asimov's. Reprinted in a few major retrospectives (even one named after this title) by Sheila Williams, Charles Ardai, Isaac Asimov, Martin H. Greenberg, Bruce Coville, John Gregory Betancourt, Norm Sherman and others uncredited. It won a Hugo and Asimov's Reader Poll awards, was up for the Nebula and Locus awards. Audio and online.
I've read this story I don't know how many times. I thought it was interesting enough, but why was it a classic?
Sometimes stories break rules successfully. This is one of them.
First, we have an ordinary joe, the narrator, who needs a job while in high school. He doesn't even really like school except to see friends and girls. He's rootless, wandering through life, doesn't know where he belongs, but the money's good. This should fit a large majority of readers who went to high school bored, not seeing it as a ticket to somewhere better. Typical.
Next, we have the diner, Harry's. He's got stringent rules, but they can be broken without consequence. What's cool about this ordinary, off-the-beaten-path joint, is that it has weirdness. Who doesn't want a little spice and adventure in their everyday, humdrum existence? People come in April dressed as if it were mid-winter, others as if it were a muggy July or August. Some don't even look that human. But they are--just from different universes. People, the unmoored, pass through, knowing they can get stuff at this diner without questions. In other words, it's a kind of Cheers except nobody knows your name.
(Side note: Three women walk in shirtless--having come from a vastly different society. Having been written near the hippy generation when it was cool to be so liberated, this probably struck a chord with those readers. The newer generation may have pulled up short, thinking it gratuitous. I prefer my magazine covers not to flash gratuitous boobs myself, but I understand where the earlier generation is coming from.)
Next, we have what lies beyond the diner, what it represents: other possibilities. There's a world out there, just for you, the wanderer. It's just what you've been looking for, but it will take some looking. But... you might not ever find your way home again.
Read in this way, while it doesn't have the supposed necessary ingredients of a good story, it's a kind of wish fulfillment, but not just starry-eyed wonder. There's a cost. Will you pay it?