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!
By way of providing feedback from someone who recently posted for the very first time: while I don't think the new rules would have prevented me from submitting my three extant posts, I do expect that I will be more cautious in the future than I otherwise would have been. (I'm well known to suffer from karma-loss-aversion.)
I'm not opposed to the change, however -- votes on posts should be worth more than votes on comments. (Also, my karma more than doubled when the change was made.)