For my money, a lot of the magic is in the smartnote categories; knowing what fleeting, literature and permanent notes are is the basis for recognising and almost automatically doing what you should be doing now.

This is similar to the gardening categories I use: cold compost (annual weeds), submerge (perennial weeds), stones, rubbish. You need a container on hand for each of these as they turn up at random. The benefit of this is that you eliminate the decision-making process which interferes with a gardening task and it's associated potential flow state. This is very much like the cognitive outsourcing aspect of GTD.