Scalability requires mathematical reasoning. From implementing 'you may also like' in an ecommerce store to pulling up a combined list of your friend's pets, I'm reminded over and over again of the gulf between non-mathematical programmers and ones with a basic undergrad math education.

edit: ...along with spending months doing homework on algorithms and big/little-* notation, and learning automata theory, of course. I don't mean to oversimplify.