Probably the one thing that causes repeated mediocre and poor paintings is what I call the "image trap". The artist is so concerned with the images in the painting that image take priority over sound composition. Things get painted as individual pieces in local color rather than being related to the whole, considering how color masses behave in light and shadow.
A term I'm using these days is "raw image". Not very elegant, but to me it conveys images out there before they're ever touched by the artist. We select images within their environments to carry our content (or as some say, concept). They are why we're doing the painting to begin with, but if those very images are painted out of context with how light and shadow behave the painting feels disjointed, fragmented, disconnected.
There are many ways to put images in a creative context which visually connects them and, at the same time, enhances their purpose. All these methods are the heart of what good composing is all about. Any single set of raw images contains potential for a profound, mediocre or poor painting. That's why the painter learns strategies to stay out the image trap, therefore strengthening the quality of the painting.
During the next few days, I will talk about staying out of the image trap and keeping a painting connected without making it boring.