I agree that instructors should have solid knowledge, both depth and breadth of dive experience, good teaching skills (including time management and interpersonal relations), and be passionate about diving. I'm not sure I agree with what I think is the OP's suggestion that someone with 150 dives is definitely not ready to be a DM candidate. I think I understand why he feels that way, and I used to feel that way too. Now I don't, and here is why: I think a DM should approach performing those responsibilities in the same way that any diver is supposed to approach any dive -- you should dive well within the limits of your training and experience. If a person who only dives at a local quarry is sufficiently knowledgable about diving there, has the right skills & behaviors and completes DM training, I believe he could do a good job working as a DM on dives at the local quarry. That doesn't mean he should move to another state and immediately begin work as a DM on a dive boat, because those conditions are outside not only his DM experience but also his personal dive experience. If you want to DM in new conditions, first you have to become proficient personally. Then you can incorporate the DM activities.
Each person learns and develops skills at a different rate, and therefore I don't know that any set of minimum standard prerequisites will guarantee every candidate for DM or Instructor is ready and will be good when finished. But we have to start somewhere, and hope that folks who train DMs and Instructors will guide and coach individuals who pursue these certifications to ensure they don't put other students at risk.
This summer I decided I wanted to get a DM certification not because I wanted to work as a DM, but because I wanted to learn more about dive physiology and improve my dive skills. I love learning. Well, I will tell you that the DM class kicked my @$$, and I have not yet completed it. And that is ok, because I learned a lot and I will likely complete the class sometime within the next year. And that still does not mean I will immediately begin working as a DM any class any time. I don't think I'm ready yet. But I do think it was valuable that I could take this class now because it has helped me realize what I am good at in diving and prioritize what I want to work on next. I plan to never stop learning, regardless of whether I am just another diver, or become an instructor or a course director.