alt Hacker NewsThat's because you are thinking in the way you would with conventional cad systems, you are doing your operations sequentially a bit like a sculptor would: take a blank and then remove bits piece by piece. The trick to working effectively with OpenSCAD is to generate what you want, not to iteratively chip away at something.
I have found that a combination of the two approaches works best for me. In some cases it is easiest and most practical to combine (union) shapes to get to what you want, and in others it easiest to subtract (difference or intersect).