Photoshop, is one of those things that is exactly what you make of it and how you use it and we as people have taken it to an extreme level. Photoshop doesn't have to be used for unnecessary purposes, but for some twisted reason we still do. Perfection is unachievable, yet some make it a personal goal because of manipulated perception of beauty.
I find the element of space and perspective really interesting, therefore think that would be my favorite I believe. I think that it makes you look at the picture in a whole new way, as if you were a part of the painting itself, which I think is an amazing thing to have in a painting.
Although my favorite principle would have to be rhythm and movement. I love looking at a piece of art and it is as it goes together fluently.