Composition.

Don't let ANYONE tell you it's a rule made to be broken: that's shorthand for them being crap and throwing away received wisdom in order to be crapper.

Thirds, edges. That's it.
Harmonitas, claritas, quidditas, etc: ie, for a Real Explanation of aesthetic theory, go to Aristotle; or, if in a hurry, Thomas Aquinas. Oh alright then, Chapter 3 of Portrait of an Artist as a Young Man (Joyce) contains a graphic and potted version.

Repeat numbers, shapes, textures and tones for monochromes: less is more, usually. If you own a Leica you can get away with being arty AND useless for a few forums' worth, on grounds of "texture" or "exploration", and a hi-res, ill-composed shot of a defocused turd will win a second look on a Leica.

But only a second. Keep it simple.

Rock on f22!