A good painting has two elements that make it interesting and powerful.
1. The subject must be deeply felt by the artist. That doesn't mean that you have to love barns in order to make a great painting of a barn. It means the artist must be moved by some element that is the focus. It could be the way the light falls across the old boards for example. If the artist doesn't feel it, the viewer certainly won't either.
2. The subject has to be stated simply. Again, that doesn't mean there can't be lots of detail in a painting. It means the subject itself must be clearly stated. There should be no competition in the painting that detracts from the intended purpose. No unnecessary frills. Frills will not help a painting where there is no primary focus to begin with.