The height advantage/disadvantage isn't a small mechanic change people are upset about because of 5e rule puritism. As the OP has explained in great detail it is a change so detrimental to the 5e rule set it isn't D&D. It is a cheese mechanic available to all classes including spellcasters that affects all classes to the point it makes most classes completely irrelevant. Add the "free action" nonsense that allows disengage AND an attack for all classes is simply not D&D.
If Larian make the effort of including a plethora of classes and 90% of them are made redundant by making their class benefits available to all classes by either standing 6 inches higher or behind the enemy then they are wasting their time. If they add better benefits to classes to compensate then the game gets further away from D&D.
If they start from D&D core rules and make changes based on feedback rather than including DOS mechanics it would go smoother. I don't mind changing the D&D rules if they don't work in game, don't care at all. But they haven't been tried as yet soooo. Nobody expects 1:1 creation of 5e so I don't accept this argument.
I understand this is early in the EA so I am not concerned. If these mechanics remain unchanged closer to release it will be something to gripe about. I am pretty sure by this thread alone Larian are aware of these concerns.