Here’s the thing. The shift from a paper based GTD system to a Omnifocus or otherwise digital system has had major repercussions re review – the main one being that review becomes a) easier to let slide and b) ever more crucial. In a paper based, 43 Folders type scenario, your system is extended in space, is in a sense all visible at once, you are in a sense doing a review, or at least being prompted to do one, every time you address the system. In the case of a digital scenario however, except for the space on your screen your system is overwhelmingly extended in memory, in a nested system of files such that a given view of your system tends to cover less rather than more of it. It’s because projects, tasks, goals etc etc are inevitably largely hidden at any given view/moment that a conscious effort to run through all of them at a regular interval becomes ever more crucial.