There's something seductively attractive about one-shots. I first started designing one-shots with Call of Cthulhu, and for a very long time it was my tradition to design and run a Call of Cthulhu game once per summer and once per winter, resulting in 1-2 one-shots a year. When my blood was up I could design both of them at the same time and even tie some hints from the first into the second and give revelations that spanned both games. It was an interesting time, but I was extremely unschooled at the art and I found that every time I designed a game it was too damn big to be run in a single marathon session.
I can imagine D&D one-shots would be much the same, since a whole module takes quite some time to run. I've never actually been to a game run at a Con (though the idea doesn't appeal to me simply because of the general rules of a convention and my predilection for being in control of every aspect of the game) so I don't know how other people handle this issue.
Myself, I've been slowly sliding from pure Lovecraftian CoC games into a Borgesian nightmare space replete with House of Leaves style touches. I've found myself trimming red herrings and extra little twists of plot because I just know that it won't fit into one night. I stopped running them (due to the evaporation of my normal playing crew, my normal playing space, etc.) before I could get a real handle on what was possible in one 8- or 10-hour session.
But there is something intoxicating about one-shots. It frees you from many of the normal restrictions of design. Everything can be even more deadly, terrible secrets can be integrated right into your PC's background and you can unstopper every cork. You can turn all your dials to eleven and beyond, and still be left with a game that will horrify, terrify, or do whatever it is you want it to do.
This is mostly where I have spent time with theories as to how to scare players. What frightens them? I've discovered that the monsters of Lovecraft must be handled with extreme care, as the master himself handled them perhaps. They cannot be overused or revealed immediately, they have to be carefully horded and a real immense fear of them cultivated before they ever show up. How do you do that?
I try to establish an environment of oppressive ancientness, almost the way one might in a horror movie. Mouldering apartment rooms, old houses, and a general sense of dismalness. Following that, generally, I attempt to build the horrific nature of the creatures or secrets or reveals by using handouts and manuscripts uncovered through research (though research really has to be truncated for a one-shot, I find, since I like to follow Aristotle's Unity of Time for the most part). Only in the very climax do I ever reveal a full creature for display.
Somehow, though, I feel like the new Borges House-of-Leaves stuff I'm going to be pulling in on my next design (the first one in many years) is going to be even more horrifying than any deep one or colour out of space could ever be.