I don't think it is a matter of leaving out, as much as it is the paradigm and user interface. In my experience, it requires too much customization to set up and too much thinking to use. It's a good example of software programming the user. I think OF is limited in the marketplace by its perceived complexity.
In contrast, OmniGraffle is much more mac-like. I don't know that the designers of Things benefitted from seeing OmniFocus, but it seems likely. There is less going on, but the flow is more fluid.
In contrast, OmniGraffle is much more mac-like. I don't know that the designers of Things benefitted from seeing OmniFocus, but it seems likely. There is less going on, but the flow is more fluid.