[JKT]

I think the premise of OmniFocus (as well as any other GTD app) is that you should have at least a basic knowledge of the method and terminology of GTD to be able to use these apps to anywhere near there fullest potential, so I think this is actually very understandable (that you need to already know at least some of the jargon).

Having said that, I personally haven't got even a basic understanding of the GTD way, and I am finding it quite straightforward to use OmniFocus for my own style of getting things done (simple project planning and as a to do list). Admittedly, there are many features of OmniFocus, such as reviews, that I haven't even bothered to touch as (a) I don't appear to need them for the way I am doing things and (b) I'm not sure that my gtd style would let me use them properly anyway as I undoubtedly haven't got contexts set up the way they are meant to be set up.

I guess that your criticism is right in a sense - the Help and manual could do with some basic guidance on what GTD is and how it works* but, arguably, the counter-comment would be that you should read David Allen's book first before starting to use a GTD app. IOW, the first item you should have on your OmniFocus/iGTD/Things/etc list is to read a GTD text before starting to use the app, if you intend to use it for (near) pure GTD. I should add that to my own ;)

* Edit: and also how this is applied in OmniFocus and vice versa