Check out http://news.ycombinator.com/lists. When you visit these links, they show up at the top of the page as a new section in the normally blank area to the right of 'submit'.
Really? I've got nearly 20,000 karma points and I can't downvote submissions. You don't have 500 karma points, so where did you get your information? It seems to be wrong.
Certainly at about 500 poimts you get to downvote comments, and at some stage you get to flag submissions and similarly to flag comments. There are other things that kick in with karma, such as the ability to creat polls.
But as far as I know you never get to downvote submissions.
Note that the number needed to downvote comments changes over time. About a year-ish ago I was able to downvote comments with ~200 points - today I am unable to at 330 points.