I think you would find that most of us stray a bit from the one true GTD way, whatever that is. In my opinion, OF does a good job of supporting a GTD working style, but the pieces provided are suitable for doing other things. At this point in time, there are some areas which might be less convenient for the non-GTD user (no built-in support for tagging or priorities, to name two frequent requests, though Omni has promised future support for a flexible metadata column which could support those concepts), but in the final analysis, don't all task management doctrines boil down to identifying the work to be done, organizing it, and doing it? OF provides a substantial set of tools for organizing tasks and provides Applescript support that allows extension and customization of the built-in features.
Having said that, I think there are cases where it might not make sense to use OF. If you aren't willing to use the context notion at all, even just assigning the same context to everything, you might as well use OmniOutliner, which is a powerful organizational tool in its own stead. OmniFocus evolved from a tool called Kinkless GTD that was an OmniOutliner document with lots of Applescript. Personally, I think the context notion is essential, and can't imagine organizing my work without it. OmniFocus also isn't really intended to be multi-user workgroup software at the present, though with some care and understanding of its limitations in this regard some might be successful using it as more than just a personal organizer.
Some of the features I find to make a big difference for me are (in no particular order, and some span more than one category):
- capture tools -- Quick Entry, clipping service, Mail support
- organization -- contexts, grouping, perspectives, start dates, repeating actions, action groups
- execution -- Growl notifications, review support
I can't really imagine a system where I wouldn't find those to be vital. The review tool is something that doesn't really get much press, but the framework it provides for helping to keep projects on track is invaluable.