You can routinely get a hundred or so karma from a comment thread without much difficulty, if you happen to leave a top-level comment early on a post and that happens to make it reasonably high on the front page, and then go in to reply to the inevitable responses to it (excluding an outlier comment of mine, I know it's possible to get over a hundred on a single comment as well). Not only do links earn you karma at a reduced rate, they are also nontrivial to find: for any "news" you need to beat out the dozen other people who'll submit the same thing as fast as they can, and for other content you actually need to find things that interest Hacker News and that hasn't been shared too much before. Plus, if you exclusively post links you miss the discussion, which is the best part of Hacker News IMO. Twitter or Reddit or Slashdot do link aggregation too.