Sometimes I finish tasks and I have absolutely no clue why I just did that. It was on my @computer list, along with a gajillion other things, so I know it's supposed to be moving a project forward somewhere, but I have no clue which one.
I've completely failed at projects on more than one occasion because I did the next action, had no clue what to do next, and moved on to the next thing on my list. I got like a hundred little next actions done, meaning I crept forward incrementally on all my projects, but missed the deadline on the ones I should have been working on.
Before GTD I would have just hunkered down on whatever project was most urgent and gotten it done. I was stressed out, but at least I was getting projects done on time.