You can't downvote on stories, only flag them, even after you get the downvote button for comments. And FWIW, I've managed to get 550+ karma without submitting

You can't downvote on stories, only flag them, even after you get the downvote button for comments. And FWIW, I've managed to get 550+ karma without submitting any stories and only posting (what I hope turns are) interesting comments. So don't think you have to submit stories to get karma.