According to the policy, add-ons must be free of charge, may not solicit donations, and their code must be completely visible.
This means some leveling mods that charge for their use are now against the rules. Essentially if you develop a mod it has to be released and distributed for free – no charge is acceptable. And for those thinking "well, the developer could just charge for the download service then." Not true.
The complete statement after the break.