Ellobogrande made a great point about the guilt. I think I'm hesitating to dive into the mind-dump step because I know when I finish it, I'm going to have a huge pile of stuff staring accusingly back at me -- failed deadlines, forgotten projects, etc. -- and I'm going to be able to see concretely how many failures and missed opportunities there have been. Of course, they're already subconsciously beating me up every day, but facing them in a tangible way is very different.
There's a fair amount of fear, too, that goes something like, "What if I do this, spend the time and emotional turmoil to go through this whole process, and it doesn't work for me (though my own fault, not GTD's)? Then I'm still going to be disorganized and behind, but I'll have a bigger pile of stuff staring at me, and a more conscious idea what's in it, so it'll be even worse." It sort of feels like double-or-nothing, as if GTD will make things better or worse, but certainly won't leave them as they were. Maybe a person has to get to the point where he feels like he doesn't have a choice -- either get organized or give up on life.