I'm with David. You can stand by the projector and watch the film go through. You will instantly know whether your show is running right. If a problem develops you can see what's wrong and, with luck, correct it before the audience even knows. With digital projection, you have know way of knowing that there is a problem until after it has already occurred. You might be able to correct it. You might not.
If you want to screw up a person's filmgoing experience for the rest of his life, teach him about cue dots. I've had Workstudy students who were operators in my booth come back, years later, and curse me for ever teaching them about how projection actually works. Once they learn to watch a film critically, from the technical standpoint, it becomes natural to see the splices going through the projector.
If you really want to torture a person to death, you can teach them about CAP code dots.
The truth is that most people don't even see these things, much less any other defects or digital artifacts. Though I don't expect everybody to be able to analyze a picture as well as I can, I do expect people to be able to recognize a quality image when they see it. That's what frustrates me the most.