Each of us took OW..most of us took many other classes as well. I think the issue we need to address is how many drastically new skills do you pick up in the class ( plus important new knowledge critical to your diving), AND in comparing all classes taken, which ones packed so much of this in -- that it became more like a huge challenge to complete the class successfully, than just a few minor tweaks and some minor new knowledge--which is what MOST major agency classes are like, due to the nature of the "Modular" Learning systems widely utilized today.
(Saying this as an instructor, so there is a little bit of 'trust me' in this, but that cannot be helped.)
Here's the thing: We can constantly remind students they are learning new skills, or we can just set them up in a situation where they do the new skill without ever realizing they are doing a new skill. Wu wei, as the saying goes, or Upaya/Skillfull Means if you have a bent to Asian philosophy. Muscle memory works regardless of whether a student is aware of a new behavior or not. IME in fact, it works better if the diver is simply not aware that there is any way but the way they are acting.
At the end of a properly taught open water course, OW divers have learned such a drastically changed set of behaviors that if we remind them that they have learned a drastically changed set of behaviors constantly they could get intimidated by the very list of new skills.
Using OW because every diver has done it:/
No student of mine would ever call the OW course I teach hard. It is not because it is not full of skills, multiply repeated, with quiet,in trim neutrally buoyant divers as the end product. It is because I rarely announce what they are doing
as a skill because I have found that diving gets harder the harder a student tries at it, and if they are anticipating evaluation (which is in fact what is happening), rather than seeing it as trying out new behaviors for themselves, they will do worse because diving is harder they more effort is put into it. They don't know they are constantly being evaluated. They are.
The use of the personal pronoun is not saying there are not an huge number of non-ego driven instructors who also teach is such a way that the divers are unaware of their progress, and the instructor becomes as much as invisible, it is just that I rarely get to see them, and never get to talk to them IRL.
There are three factors that an instructor can do to make skill progression natural or 'hard':
1. Focus on something other than what students are doing in the water. (My thesis is Lectures is usually where the energy is transferred to.) We have seen a couple examples in this thread not provided by me, where inattention to the water has made the class hard.
2. Confuse bad organization, and insufficient coaching with teaching a complete course, and having students think that hard is somehow a compliment about the course or instructor. We have seen at least one example in this thread not provided by me.
3. Make the student aware of the level of constant evaluation going on, and contribute to student's failure to gain a sense of mastery at a skill because of the constant awareness of evaluation. Telling someone that they must maintain buoyancy, right now, in this space, for evaluation, makes it far less likely that someone will be able to do so. This goes to your quoted comment. Simply put, a progression can be natural, or difficult. Because as instructors, we are trying to ingrain behaviors to be exhibited at every moment on every dive and flow naturally as part of every dive, setting up an evaluation stage is counterproductive to both the long term goal of ingraining diving behavior and the short term goal of evaluation.
IME, all three of those factors are driven largely by ego on the part of the instructor, in some sense:
1. Lectures are where instructors are in complete control and they tend to speak as oracles, rather than pointing divers to further knowledge. There is some basic knowledge necessary for the activity, and there is some current consensus in the diving community, but we instructors are only getting received knowledge (and can change at any time). Pointing the divers to the real experts is a better plan because then later they will look to the original sources for the newest knwoledge, but then we have to take ourselves off of the oracle platform, and just point the students admiration somewhere else.
2. Because a 'hard' course is somehow a compliment in some instructors mind they do not see student failure as a reason to turn their focus on organization, and coaching. With better organization and coaching any course gets easier, and the 'compliment' of course being 'hard' no longer is given, so these instructors are not driven improve their courses, because they would start being told their course was something other than 'hard'.
3. By simply pointing divers past the course constantly, and making skills happen naturally in the course of diving, it is hard not to have students think past the course. If students think past the course, then their allegiance is to the activity of diving, and not to the instructor or the agency, or course. For an instructor who is focused on themselves, it is crucial to maximize briefing, and constantly talk about the skill because they have to keep focus on instructor agency and course. If the skills evaluated just become part of the fabric of the activity of diving, then students are not even aware of the milestones they are passing. Again, the instructor becoming invisible is not an easy thing to swallow.
Anyway, just wanted to throw a little perspective out there for context. Also, come check out Norcal diving sometime - we'll show you a fun time. 
