"I know that medical schools value quality over quantity, but there is always the urge to list every last activity."
EXACTLY! Especially since you can't rank your most important activities first. In retrospect, I shouldn't have put so many down on my AMCAS last year. The poster presentation overshadowed the publications, same thing with the one month stint with Habitat. Don't put down that you attended meetings for the premed club. I also wouldn't put down Who's Who, The National Dean's List, Golden Key, Phi Eta Sigma (Phi Beta Kappa definitely). Of course, this is assuming you have excellent extracurriculars.