I didn't go to school (ok, I went for 6 months and dropped out...). I was mentored by quite a few ruthless old farts...
I was taught:
Order, Variety, Contrast, Symmetry, Tension, Balance, Scale, Texture, Space, Shape, Light, Shade, Color as well as Proportion, Conspicuity and Relation (to the observer, architecture, brand, or connection to what is being promoted)
The common mistakes I see is... designers do not connect their layouts or design work to the branding, architecture and code restrictions. I can knock out pylon sign designs up the wazoo in record time. But if it doesn't relate somehow to the architecture, there is a disconnect between the sign and the building and those looking for their destination. Or adding typefaces and color to a wrap, banner or panel and do not work well with the existing logo, or code signs that are non-compliant as well as not cohesive to the project. This can be taught...