My guess is that we're seeing the results of the tech blogger echo chamber. These self proclaimed experts need methods to legitimize their existence and quantify their awesomeness. What better way than a big fat number on the one end-all-be-all site? See also: Scoble was biggest contributor on Quora.
Of course, it's all ridiculous (for now). Most people in the real world have better things to do than troll around on Q&A sites hoping for karma. Until someone figures out how to convert karma into a competitive paycheck, this is all nonsense.
I'd be hesitant to hire someone that had huge karma on any site (including HN), unless they had real projects to show that their passions include shipping rather than just talking real pretty about it on the Internet.
Someone can be clever and knowledgeable, and still be a poor employee or co-founder. (And, my least productive times are often when I'm most interested in what's happening on reddit/HN/etc.)
Decently large karma means there would be a lot of comments to browse through though which can give you an idea about the person. To a point be knowledgeable on startups and the tech scene is an advantage but yeah there is a point where time is better spend getting something done.
My comment should be taken with the knowledge that I have pretty big karma at both reddit and HN (and StackExchange, though I've never browsed or considered signing up for quora). Obviously, I like to think I'm a pretty useful person to have around in a company. So, my comment is definitely one of balance rather than "don't hire people who talk a lot on the Internet".
Karma the number will never be converted to a competitive paycheck because there are too many ways to game the system.
What is good about sites like StackExchange is that they encourage public discussion, and knowledge sharing. So if you have a StackExchange account, an HR rep doesn't have to just take your word for it that you're a well spoken, helpful individual---they can peek over on StackExchange and actually see your answers, comments, and questions. The karma attached to them is a meaningless indicator of popularity and not quality.
Of course, it's all ridiculous (for now). Most people in the real world have better things to do than troll around on Q&A sites hoping for karma. Until someone figures out how to convert karma into a competitive paycheck, this is all nonsense.