If someone starts a post with "Rationality is wrong..." or similar, I'm much more likely to downvote it than if they start it with "I've got these scenarios where standard rationality techniques seem not to work..." To this extent at least the presentation of the ideas matters as much as the content. So I hope that these rules will cause people to present their ideas more cautiously, while still posting experimentally. If you are thinking of posting something controversial, it might be worth seeking advice from the other users who you think will be interested.
Basically, I think downvotes should work as an "utter stupidity" filter.
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!