There are upvote/downvote buttons -- I'm unsure why any other criteria is needed.
I used to avoid voting on comments on my phone for a while because I was afraid I'd end up downing with my fat fingers but they've fixed it with an undown link when I down vote. Maybe they've relieved me from the responsibility of down voting stories?
I don't know how I feel about down voting because there is always a desire to down vote dissenting opinion.
tl;dr that's for comments afaik