What do you consider adding attachment download rules?  I mean mainly that, for example, a given user cannot download an attachment in any section until he writes 5 posts or the appropriate time has passed since registration?

We already have that feature. What you would do in order to implement this, is to remove permissions from their current group, then create a new group which has permissions to download. Then create a promotion rule which adds that as a secondary group (or moves to it) on reaching a certain number of posts.