If one throws an hojillion low-quality posts up in a day, a few of those will be ranked +1. That grants frequent posters a ludicrously high karma score for what amounts to little overall contribution to the 'community'.
In a way, it's almost an MMORPG -- the longer you stick around and 'grind', the higher your score gets.
If you can't be one of the first commentators, it's best to reply in a way that puts your reply as close to the top as possible - replying to anything "below the fold" is a terrible investment.
[Aside: yes, I'm ashamed to care about karma enough to think these things through, but somehow, I just can't help it]
Unfortunately these are hard problems. I've been thinking about them since early Reddit and still don't have solutions.