You need a few points yourself first (no idea what the threshold is). Post some interesting items or join in the debates in the "comments".<p>I think you can probably see that this is a guard against "sock puppets" being used to promote spam posts.
It looks like on this site, anyone can express an opinion of agreement with an up vote but only some who have earned it are allowed to express they disagree by using a down vote. Anyone can comment though.