I think you've hit the nail on the head here - potential. However, have you noticed how many questions are asked (particuarly homework-related ones) by monks who only joined the Monastary a few seconds before?
Maybe a way to increase the quality of PM would be to disallow novices from posting? This will mean new members would need to visit the Monastary reguarly for a couple of weeks before asking questions. Hopefully in this time they will have read the FAQs, tried Super Search and learned a bit about what PM is about, instead of joining and immediately posting "I need this URGENT!!!".
Just my £0.02,