One of the things that helped me with GTD "implementation" was to realize that items on my @NA lists are simply reminders and nothing more.
Like a previous poster mentiond, GTD doesn't give you any more work than you already had; simply, it just makes you aware of everything that you've already comitted to. If, after doing something like a mindsweep, you feel overwhelmed you should ask yourself if you've overcomitted.
It's funny; what really hit the nail on the head for me was when recently I had a glitch w/my palm where all of the items on my task list got deleted. This did two things:
1). Made me realize, now that I had everything written down and out of my head, how dependent I actually was on the system.
2). Made me realize how much more stressful it was trying to think of and remember all of the the things that I comitted to rather than having it written down somewhere and out of my head.
Realize that no system is going to do your work for you. I found the FC system to be even more frustrating. I'd spend 15-30 minutes each morning "planning" my day, creating my "A, B, C, 1-2-3" list only to go into the office and have my boss completely turn things upside down. Their "just say no" approach to overcomittment isn't really functional in the real world. At the end of the day, I'd have to spend another 15-30 minutes feeling guilty that I didn't get through my list, and "forwarding" every thing that I didn't get to that day. This doesn't happen with GTD. My comittments are there, and if I can get to them I get to them. If I don't I don't; if I don't, they're still there reminding me that I have an open loop out there (I don't have to waste any of my "Psychic RAM" trying to keep tabs on them).
One thing that I will concede with the system that I've had trouble with is installing a sense of urgency to my @NA's. There is nothing, other than my own internal "doorkeeper" to tell me if one item is more important than another. Oftentimes, there will be things that I don't want to do that are important; it's too easy to say "no".