As recently (re-)suggested by Kaj Sotala, posts now have much larger effects on karma than comments: Each up or down vote on a post is worth 10 karma.
Negative votes on posts have had karma effects all along, but for some reason Reddit's code imposed a display cap (not an actual cap) of 0. This violates a basic user interface principle: things with important effects should have visible effects. Since this just got 10x more important, we now show negative post totals rather than "0". This also provides some feedback to posters that was previously missing. Note that downvoting a post costs 10 karma from your downvote cap of 4x current karma.
The minimum karma to start posting has been raised to 50.
Thanks to our friends at Tricycle for implementing this request!
I would actually kind of like it if it did cost karma to downvote. It would mean something to me when I downvoted a post that was irrational or in my judgment out of line socially.
What would be even better is if it were possible to downvote comments multiple times at an exponentially increasing cost. There are some comments that are in such contrast to my preferences that I would be willing to spend 63 votes (1+2+4+8+16+32) to down-vote 6 times. This would sometimes reduce the need to do 'costly signaling' in the form of comments with strongly assertive and direct criticisms.
I can imagine myself thinking "No, that is blatant bullshit but I know if I call you on it you will throw more bullshit and drag me in to your drama. I am willing to pay 31 karma to discourage the bullshit without the (minor) physiological and psychological stress of engaging in social conflict with unreasonable people."