About 10x more people comment than post. About 10x more people vote than comment.

It's easier for a user (especially a brand new user) to write some snarky one liner comment on something they read than to compose a new, multi-line post in a blank window. We think that rewarding this kind of "drive-by" participation more is a valuable improvement to increase engagement and retention, and to generally incentivize more content and sharing of opinion (which offers more value to readers in turn).

Also, comment threads are fun.

(I was one for a 50/50 split, personally. Comments make the site.)