I tried to explain something to my musician son the other day. I had a hard time. The concept is nebulous. You have to ken it peripherally. If you try to look right at it, it can't come into focus. It's as much art as science. Hunch-y, really. I tried to describe to him what makes a song have "legs" -- a term we used to use to indicate that a song is potentially useful to a performer by its very nature.
OK, so the Clutch Cargo of Country™, George Jones, had a big hit with this one back when Minutemen still rode dinosaurs to the Post Office to use the only telephone in town. That fact alone isn't going to cut any ice at the disco, brother. Besides, he didn't write it. He had to spot the legs in the song in the first place. If you want to glom onto the esssence of the song, and milk it to go along with your own performance cookies, the song needs to have legs. It's got to be the framework for entertainment. It has to allow others to produce their own artifact, not just trade on the previous artifact.
The wrong people have to be able to "get over" with a song with legs. The sum of the component parts have to add up to more than the parts themselves. So you become a kind of vivisectionist, taking songs apart to see what makes them go. But just like taking that frog apart in science class, the frog doesn't work anymore if you take it apart. The animation comes from somewhere else. To choose a song that's going to have legs, you have to understand the frog well enough to replicate it, but you can't kill it while taking it apart. That's why it's so hard to know what's going to work.
You had a disc jockey at your shabby, expensive wedding because you didn't want music; you wanted a list of cultural artifacts, laden with the context of your memories of what you were doing when they first came out of the radio. You wanted to eat at Musical McDonalds ™ because you wanted to know exactly what was on the menu before you entered the building. You didn't want to rely on a chef, even a world renowned chef, because improvisation is fraught with peril. Something might happen, and your wedding would be on YouTube for all the wrong reasons -- the only reason anything is on YouTube. To perform a song that has legs, you have to make the audience forget there's another version of it they prefer for a little bit.
You're on to something in your selection if a wave of nervous laughter passes through the audience at first, finding, perhaps, a delicious irony in the resurrection of a hoary old thing, and then the dead silence of rapt attention has to follow it.
So you search without looking directly at anything, the way a man searches for a mate in a bar. Sometimes you find exactly what you're looking for, and the audience thinks to themselves: What a cute couple they make.