I hadn't thought about Tags before this thread. I hadn't thought about them before it. I was not aware that some software supported it, however you could just make something in the subject line of the task to simulate it (e.g, t+easy = this is a quick and easy task, requiring low energy).
I would guess the advantage of making it NOT a separate category or PART of a category is that it's likely to be cross category. In the above example, any place/time/equipment context could have an "easy" task in it, while it would seem silly to have just a list of easy tasks, 3/4 of which you couldn't do in any particular situation. You could add it to the task (e.g., @CompWrkEasy), but then you end up with #Categories (Time/Place/Equiptment) X #Tags lists.