Like someone said earlier...PADI teaches the basics for OW. I think it is up to the student to use some judgement on their own part. If they struggled during the OW checkout dives, they know they did. They should know they arent ready to go down and check out the wall dives in Cozumel. But is it any different from getting your drivers license? There, you take a written test, and drive for 10 minutes with someone in the car with you. Then you are on your own. That doesnt mean you are ready to hop in an 18-wheeler and drive across country. I know when I did my OW class, I knew I had a lot of work to do to get better. I asked my LDS if I could go with them on other OW classes, and hang out behind the platform working on hovering while they do checkouts with the rest of the class. The LDs saw I was commited to making my self a better, and safer diver, and never charged me for that, and always let me go. I didnt even have 30 dives in yet, and I had a reg free flow on me at Mermet. I didnt panic, I signaled to my buddy I was having problems and we aborted the dive. Sure, PADI OW class taught me what hand signals to use for that, but common sense taught me it was time to end the dive.
Now, for other PADI certs, I do think those come a little too quickly. I dont think you should be able to do AOW until you get some dives under your belt. SInce one of them is a deep dive, I think you better have the buoyancy thing figured out, otherwise you can get in trouble in a hurry. And it better be second nature when you try and do the Navigation class, otherwise you are going to either be at the surface before you know it, or going face first into the bottom while you are looking at your compass.
And as far as DMC, being able to get in with only 20 dives is crazy. I just talked to my LDS about this last week. I want to get into DMC, but wasnt sure if I was ready to start it yet, and I am at 76 dives right now. (72 of them this year alone.)
I guess what I am trying to say is PADI, or any other group can set whatever standards they want, but its common sense and up to the diver to decide what they can and can not handle.