I think you are onto something Tom. To follow you, one area of concern I believe to be germane to the problem is copyright violation. Since there exist no "real" taboos on the web, copyrights have no meaning until the owner takes legal action, and even then, results are inconsistent.
I mention copyrights because it is easy to violate these, even unknowingly or unwittingly so. Even though I have only been here a short time, I have noticed requests that involve putting one person face or head onto a famous person's body. It really does not matter if work involves a celebrity or not, but it is a valid concern that a person's photo likeness is used without their permission or without the original image owner's permission.
I read through all of the policy and guideline posts made by the admins, and none of them discuss copyrights, trademarks or other legal issues surrounding requests for work, be it paid or free. Look at deviantART. Not only does each member have to agree to the terms of service and legal terms, they must also acknowledge these terms with every image that is uploaded. No such creature exists here.
I know that the PSG rules say an admin or equivalent must approve all paid requests, but that does not cover all the freebie ones that slip into other sections of the overall forum. This opens up the website to possible legal action. I can tell you from experience that every website I have had to send a cease and desist order to violators of my copyrights, each of them took immediate action to remove the offending content. I have had to send these C&Ds to eBay, deviantART, Go Daddy and to a popular custom motorcycle builder who split from his dad's group. They all took immediate action because none of them want to end up in court.
I am probably overstating this issue, but I believe it to be an important one that should receive top level consideration in order to protect the website and all members who may be unwitting accomplices in copyright violations. I cannot say how to institute any such safeguards. I just wanted to mention this important item.