When you're right into this hobby, you really see how clueless most people are about it, even if they are casual gamers. I don't know how much more clearly I can tell people that "this machine plays arcade games only, that's what I built it for, no console games". Then I say, "so, what do you want to play?" I've already lost count of the number of times this question has been answered with "do you have Mario Kart?" or something similar.
Other than that, I think it helps to have a game list ready that has all the fluff removed (the 4 million Mah Jongg games, the crappy trivia games, etc.). Also, keep a running short list of games that tend to have pick up and play appeal but might not be widely known. For example, everybody knows Pac-Man, Galaga, and Frogger but not everybody knows Mappy, Pengo, and Amidar.
Another thing I find keeps interest is get people using the different types of controls. Trackball games on my cabinet are a hit because so many of my friends never had the chance to play them back in the day, and these days everything is d-pads and analog sticks. People really seem to like Major Havoc and of course Centipede. Then there is the obvious thing, playing games like Golden Tee, Shuffleshot, and Shuuz, and getting the smack talk going strong.