I've been sweared at by forum members for requiring x posts to use something. Get used to having crap flung at you constantly; that's how you know you're a true webmaster.
Personally, the only time I will enforce that type of rule is to prevent spamming eg they have to have so many posts before being able to advertise or something similar. Otherwise forcing users to participate creates spam posts that are usually worthless to the overall community just so they can access a feature IMO.