Flat Design:
Flat design is still something new to the digital art scene but with the way it's going, it'll revolutionize the scene. I believe flat design is one of the more important side of designing because it also goes with the less is more idea and it's simple enough to not stand out too much.
Less is More:
The simpler the design the better. A lot of designers get lost thinking their designs don't have enough of something and keep adding things until the get satisfied but because of this they lose out on a golden opprotunity. Sometimes instead of adding more to a design it would be better to retract some stuff from it.
Spacing:
Spacing is one of the most important things in design, it affects how the reader perceives the design entirely. With spacing, the whole design could change completely with just the rearranging of a few things.
Emphasis:
Emphasis is crucial in designs. Emphasis is the part that sticks out most in the design, or the center of interest, and how it sticks out from everything else. Without emphasis many designs would just be shallow and wouldn't be as popular as they are.
Harmony:
Harmony is the arrangement of elements to give the viewer the feeling that all the parts of the piece form a coherent whole. Harmony is how well everything goes together. If you have the right harmony then the design will work great, but without harmony the entire design would just be out of wack.