Quote by GirlyWorld
a) You have to have completed a certain number of fields in your profile to be able to a) post in the forum, b) enter the chat rooms, c) rate stories, d) etc.
b) You have to have accumulated at least X number of friends before you can carry out task Y.
I've been in this lovely place over a year. I read a bit, write a bit, lurk a bit and have never posted in the forum until now. I have only completed a few rudimentary profile fields and have ten friends. Under the proposed restrictions, I presume I'd have been unable to answer your post? Well, perhaps one of the credibility triggers might be to have had something published, so I may have been alright. But some people are predominantly consumers. They read stories, enjoy the discussions, and then one day a forum post or story appears that galvanises them into action. They feel they just HAVE to post, write or rate because the content moves them in some way. So they excitedly register, go to participate, and the first thing they get back? "Sorry, you can't do that: you're too new. Jump through hoops X, Y and Z and we'll think about letting you play."
For legitimate users, that takes out all the impetus and drive of deciding to be part of the site. I'd be thinking, "Too new?! I've been reading stuff here for aaaages," and would be put off.
For potentially unscrupulous users, posing a set of challenges before being able to contribute might be just that: a challenge! Filling in some profile fields and collecting oodles of friends by bombarding existing users with requests is easy. There are probably many users here who see more friends = better, so they'll accept pretty much anyone. I don't have the numbers, it's just a hunch.
Quote by GirlyWorld
The Account Credibility system... would be a 'behind the scenes' mechanism to allow new members progressive access to the site's features.
That's fine in principle, and it might work to some degree. But you should still make it clear what the rules are so you set the expectations of the people who want to become part of the community. By declaring such rules you disclose the mechanism(s) to bypass them and risk discouraging well-intentioned members from joining. It's a tightrope. Some trolls might well be lazy, but I fear people intent on mischief won't be deterred, and may well be attracted by the challenge. These are the types of people admins will spend a large portion of their precious time cleaning up after anyway.
Unsociable people are a scourge that need to be dealt with appropriately, and sadly I don't know the best way to achieve that. But I suspect raising the height of the walls to the garden of Eden will not significantly reduce the number of snakes that slither through the gates.