"You can only downvote comments once you have sufficient karma. The required level rises over time to account for the karma inflation caused by an increasing userbase."

"high average score on your comments will give your comments preference in the search order (this is part of the ‘secret sauce’)."

"If there are no real big benefits to having a good or even excellent karma you might want to conclude that it doesn’t matter at all, but that is definitely not the case, since having ‘bad’ karma (< 0) will negatively impact your ability to interact with the site."

The italics in the last one are mine since I'm not entirely sure what that means.

I have also heard that at some karma level HNers get to change the colour of their topbar. How jolly!

Do we have a comprehensive list? If the levels are subject to inflation then perhaps a correspondingly definitive "price guide" can't accompany it, but any information would be interesting, out of date or not.