I know where most of my deficiencies as a painter/modeler come from, it's mostly is from my list of "I don'ts"
I don't work with green stuff much, not for added details, nor for simple things like filling in joints and gaps, unless they are extreme.
I don't paint any of my basing materials, I instead keep a wide variety of stuff to base with around, tallus, ballast, cinders, and different colors of static grass. Basing is always an afterthought, and hardly ever part of the modeling itself. I'm going to have to see what I can pick up in a larger quantity if I'm going to do an entire army. while I have lots of simple flock I now prefer static grass, and I like the look of mixed media on a base.
I don't wet-mix colors, I prefer to keep a wide variety paints instead. This can hamper some subtle color changes.
I don't use a palate, preferring to paint out of the tops of paint pots, which means that I have less control over the consistency of my paints.
But I know that every time I can hurdle one of these "I don'ts" I improve as a painter.