Not to be too self-serving here, but this issue is what prompted me to write this article:
http://startingstrength.com/index.ph...s#.U36YC1hdXBw
While the whole thing spelled out point-by-point is too long for an elevator pitch, you can use it to create a short "This is why we do what we do" speech. If you wanted to make it REALLY short, you could simply say something like "Full range of motion barbell exercises, done with slightly increasing weights as often as possible, are the best way to work the entire body as a complete system, which improves all aspects of athletic development and increases health and functional capacity better than any other tool or system out there." If they want to know more, you can get into details.