Some say: it most deliver on a certain amount of words (750 words and 3 images), while others say: make it lengthy and lose the concentration of the reader before the hit the end.

Some say: your command of English language most be massive, while others say: we are not native English speakers, so how do we swing our word control in English Language?

You mentioned almost every point of view my friend.

A great post in my best definition should be easy to understand

For sure my friend, agree with you.

What do you consider a qualitative post?

My answer is same as @crypto.piotr's answer - they not only read it but also share their thoughts on the same.

The post become quality post not based on the images, video, number of words - but it becomes quality post when people not read it but also understand it. I don't consider words limitation for the quality post - even sometimes single sentence or 1 paragraph speaks thousand things and the post made with 1500 make no sense. When people understand it, get inspired/motivated, start to relate them or their conditions or take their few valuable minutes to what it is about then it will be quality post for sure. When people are sharing their views/ commenting honestly on post consider it's quality post.

Like your post - see I am sharing my views and just writing things whatever I am relating with it. Because it forcing me (in good way) to share my views.

So I think what you write here is quality post  😌
Great article my friend. Wish you great success here :)