Two quick comments on this very interesting thread:
1) I don't think GTD demands "linear" planning, nor does OmniFocus. GTD is merely a way of getting you to place your intentions in plain sight where they will be actionable, and OmniFocus in turn is merely a listing & recording tool.
When you visit a project so as to generate some sort of action & keep it moving forward, that's planning. And how you do this planning is up to you: a sketch pad, a mind map, etc. Moreover, there's nothing to say you can't come up with ideas the old-fashioned way - in the shower, while driving, while walking, etc. Also, I like Allen's idea of "natural planning" - it's anything but linear.
If you like paper better, there may be other reasons why this is so. But the why's don't matter; just stick with paper!
2) GTD doesn't address a few topics that many people find important: priorities, values, and (relevant to this thread) avoidance, whether deliberate or unconscious. As dijk182 has observed, coming up with just one next action for a project is a great way to slice big scary projects down into manageable bits that can actually be done. But what if you stop creating next actions - or stop doing the ones you've created? Something else is going on, and that something else is avoidance.
I find "avoidance" to be a more useful term for this phenomenon than "procrastination" - it's less prejorative and more psychologically accurate. Avoidance can consist of making lots of lists but never doing anything; reading still another book on preparing or planning, instead of actually (gulp) doing; fiddling endlessly with software, the Web, & computers (a growing addiction across the world); creating distractions accidentally-on-purpose; etc. Clearly a tool like Omnifocus can exacerbate the problem if not handled with care.
A good book on finding ways to get more work done, enjoy life more, and procrastinate less is
"The Now Habit," by Neil Fore, available in paperback.
A good book on learning to make good rather than "perfect" choices, so as to do more and agonize less, is
"The Paradox of Choice."
And a good program for anyone who's really struggling with avoidance, and who yearns to pursue their true values more effectively, is Acceptance and Commitment Therapy. Check out
"Get Out of Your Mind and Into Your Life."