grades, first thing they look at are your grades and academic rigor. if your academics dont measure up to a certain standard unless you have an awsome hook of some sort, they wont consider your app. (thats from the ivy rep presentor)
The most important factor is your entire academic record (SAT+SAT2+GPA+Class Rank+Strength of Schedule+High School Quality). Everything else (essays+ECs+interview+letters) tells Columbia what sort of person you are and is important, but secondary.