I agree with Katherine’s point that GTD doesn’t add more inputs, it just systematically gives you a framework from which to recognize and process the ones you already have. I don’t think I get 400 inputs a day, but I do get about 110 emails a day and maybe another 50-100 inputs a day. Now, the vast majority of those are handled, as another poster wrote, by hitting delete, or by quickly reading and then deleting, or fielding a question from one of my staff and responding at that moment. What GTD does is take what’s left (I'll estimate that at between 10-50 inputs a day that require inbox processing of some kind) and let me handle them efficiently.
But I'll make a bigger point. One thing GTD has done for me is help me stop lying to myself. It forces me to recognize the full scope of what agreements I've made with myself and others. It’s easy to say yes and take on new things when you are only really recognizing half of what you truly need to do. In that case, you just try to ignore the inbox that is piling up or the stack of papers in your inbox or the mail piling up on the credenza or the journals mounting on the side of your desk. But you know it’s there and it gnaws at you, robbing you of the peace of mind that comes from knowing the full breadth of your agreements.
And what that’s done for me is make me FAR more selective about the agreements I accept. I say ‘no’ a LOT more now that I've started using GTD because I realize what saying yes really means. Someone asks me for something, or perhaps they send me something to read and before I might’ve thought “well, that sounds interesting, I'd like to check that out.” Now, I ask myself “will this either help me get closer towards my goals, or be a better person, or will it provide some kind of meaningful relaxation/fun/entertainment?” If the answer is yes, then I'll accept it and process it accordingly. But for most inputs, the answer is no and it gets deleted. I now delete a lot of stuff that if I had unlimited time I'd enjoy checking out but I realize there are only so many hours in a day.
It also encourages one to ascribe more value to delegation. Maybe this IS something that has to get done. Am I really the only person who can do it?
GTD forces you to get really honest with yourself, which is very powerful.