I'll repeat here what I've heard PG say before: If you want to improve the quality of HN, focus on upvoting. Find good stuff and upvote it.
Try reading the top links on the "new" page. If you click on an article and it's no good, write a comment explaining why you think so. It'll be one of the first comments on the article, and will serve as a sort of mini-review for those of us who tend to skim comments before reading articles.
Incidentally, reading comments before articles is a great way to help filter out the stuff you won't like.
If you find a good, relevant link on the new page, upvote it! This is one of the most important things you can do. Lots of decent articles disappear from HN without ever getting their first upvote.
Oddly, despite having seen an untold number of complaints about the lack of downvoting, I can never remember the stated rationale for not having it. But I think it has to do with the ease with which gangs of people could organize to downvote articles out of existence.
(You might think that claques of upvoters would also be a problem, but it occurs to me that artificial upvoting is a problem that gets noticed very quickly -- everybody sees what makes it to the top -- and can then be fixed, whereas the swift, silent, and unfair disappearance of articles can be harder to detect and reverse. It's a much easier problem to detect bad links that accidentally hit the front page than it is to detect good links that accidentally get banished to the gigantic, spam-filled garbage heap.)