What do you think is the essence of a great painting? The composition or the subject or the type of school that the painting belongs to?

You can paint the most sloppy, messy, ugly drool on a canvas, but if it has passion or drama, it will have been worthwhile.

I think the essence of a great painting is the composition, the subject, the passion of, along with the technical ability of, the painter. 

The composition, if the subject truly touches the heart, mind and emotions. Of course, if it's a good example of a certain school or movement, it might challenge the intellect instead of the emotions if the subject isn't something that really hits home. When I examine art, it's about mood and feeling. Atmosphere does it for me.

a painting that looks really good

At least two coats on the walls and ceilings.....more if you've brought cheap emulsion or your painting over a darker colour.........:)D

The message and how it affects you....it may be passion, inspiration, sorrow, fear, confusion, entertainment, humor, or reflection.  Anyone can learn the rules and technique of painting.  Not everyone can paint a message......  

beyond what a school will teach,
or from what type of School
the style comes from,
or from composition,
there is in the person
a visionary feeling that is transposed
or composed into 
a creation that we can enjoy.
  This is the essence that has been manifested.
but ....We seem to use our mind 
to define things as to to how we enjoy them.
Instead of enjoying it without definition
as to how we are to enjoy it.
     A pure feeling of joy
is when it has not been defined as something
we should enjoy in a certain framework of understanding.
     So the essence of a great painting
is the heartfelt feeling of joy,
or bliss, or understanding, that the observer
experiences.