I wouldnt say OW is pretty meaningless, however. Its goal is to teach you basic safety information and give you some introductory experience.
It seems to me that, of the 4 dives, only the last has any 'diving' involved. As I watched the last couple of classes, the first 3 dives are primarily pool exercises done in the ocean. Now, I don't have the check list but that's the way it seemed to me.
So, at the end of OW, there hasn't been a whole lot of diving. The required skill set has been demonstrated, no question about that. And the skills are important. But there hasn't been much diving. Or at least the part of diving where you swim around and look at things.
Now, with AOW, the diving really begins. Navigation will be more than a simple 'out and back' but the student will actually have time to see things. Sure, they have to watch the compass and count kicks but they still get to look around.
Buoyancy is an important topic but is really just an exercise. Still, there might be an opportunity to swim around, gliding over the reef.
The night dive will teach skills (herding cats comes to mind) but there are a lot of things to see while actually diving.
The deep dive isn't very deep but, if viz is good, there can be a great demonstration of the colors turning to dark brown at various depths. And let's not forget the ever popular collapsing soda bottle or expanding balloon. We used colored tape on a slate to demonstrate the change in colors. The deep dive is usually short but there still could be some time to see things.
I'm not sure what is involved with the other 2 dives but, around here, 3 of the 5 AOW dives are boat dives. The good news is that you don't waste a lot of time and energy kicking out a couple of hundred yards.
I realize that instructors have different ways of teaching the material. But, as I look back at my training, it seems to me that diving didn't really start until OW II (NAUI) and by the end of Advanced Open Water, diving was more about having fun doing things underwater than it was about just doing exercises.
I'm sure others will have a different view.
Richard