I've read through this thread a couple of times, and enjoyed the reflection it prompted me to do.
What are the roles of an agency? I think all of them develop a curriculum and standards, and issue certifications to divers and dive professionals. Most publish instructional materials to some extent. Some also do marketing, and develop business tools and provide business education to their members.
I'm familiar with several agencies, and each of them does some things very well, and others not so well or not at all. I have to say that I have been very impressed with an agency we often bash as providing very professionally created instructional materials in multiple media (even if they are aimed at the lowest common denominator), and also as providing a marketing behemoth and really good business education and business tools to their stores, even if some are unwilling to make use of them. These are the parts of agency function that most folks don't see or think about, but I think they are appropriate activities and should be done well.
The biggest failing I see in what we have lies in accountability and instructional quality. Published standards with some kind of attempt to describe what an acceptable performance is (as GUE has done) would help. Higher standards for entering instructor training and for passing it would help, too. And instructor requalification and reassessment (as GUE does) would help minimize standard creep, as boulderjohn has described on a number of occasions. Add those things to what we already have and it would be hard to criticize.