Having only been here for a short time myself, I think the general answer to your question would be: Hacker News is a lot more focused and has a much richer (in terms of experience, knowledge) community with a much lower tolerance for noise/novelty accounts/meme spouting/trolls.
I upvoted this because it's completely accurate, but at the same time reminds me of some of the things I don't like about HN. HN has a much higher tolerance for people constantly submitting things from their blogs for self promotion. If a person submits something self promotional on reddit and expects to escape sharp criticism from the community, they better acknowledge that they are being self promotional, and be awfully humble about it. Here, it seems to me that everyone acknowledges that everyone else is out to make a buck and deals with it.
tl;dr - HN has more tolerance for shameless self promotion than reddit as well as fewer trolls.
