My suggestion is that Karma points should only be awarded for HNers who discovered and contributed links that the community might not easily find.
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.
Karma accumulates automatically over time - the only thing quality might affect is how fast it accumulates, and even then just posting more would probably get you there faster.
If they are popular its popularity comes from good content, that should be shared here in HN. Sorry, but I disagree with your idea.
And not everything by techcrunch or mashable garners karma either; it has more to do with the currency and importance of the story.
#adjusted for local definitions of what is important, obviously.
but it isn't going to happen.
We shouldn't penalize such submissions though, that's wrong as well. They should just count less. Like 50% or only the points above 20 should count.
Or something like this:
if url already popular 10 times this week
karma = karma - 10
FLAG