First, because you probably won't believe me on the subject, read what Seth Godin has to say: No one ever bought anything on an elevator. Contrary to what anyone else has ever told you, you want time to make your proposition. You don't have that time in the elevator.
The principle is the same where a résumé is involved, right? You don't have the space to write down all of the good things you might do for an employer. Or, in other words, the résumé will not (usually) land the job for you. All you can hope to do is to whet an employer's interest in you—and more importantly indicate your interest in solving the problems that employers have. What you are trying to sell is an opportunity to meet to show that you can do what the employer needs.