I did enjoy watching this demonstration by Frank O'Cain. It's very different from the typical tutorial oriented content that floods youtube. And I believe most of the ideas can be applied to all sorts of paintings, not just abstract stuff.
Not a painter, but I do agree with the Tension/Shapes premise.
When creating my Photography-based Abstracts, I have found that mixing round/soft edged elements, with sharp/pointy elements creates pleasing results. Same with using different colors to create interesting and contrasting combinations. So it makes sense that it holds true in the painting world as well...