To Merlin's "Don't clutter the interface with excessive fields" point:
The arguement "stop adding fields and work now" can be applied to any point on the continuum from "single post-it with a task on it" to "huge multi-part project". "Don't invent multi-page notebooks, just do your work", "don't add field a, b, c, just work", etc.
I know it's possible to drown in too many fields, but lobbying to restrict the app's ability to group tasks/projects to less than (for example), 1) a particular person, 2) a particular location, and 3) a particular type of effort, feels too puritanical for such an evolving discipline.
I mean, multiple fields are what computers DO, Omnidudes! I'm fine with a checkbox in prefs allowing one to prune down OmniFocus' [viewable fields per record]. We'll all aspire to the Jedi GTD-fu skills of minimal fields displayed, but I have yet to be convinced that there's some sacred limit on number of fields per task.
A "tags" field is ONLY ONE FIELD, and can loosely allow many variations of GTD to flourish.
I think trying to cram "multiple contexts" into OF would break it. A tag field lets you tag for people, for location, even for a (crappy) implementation of priority. In ONE FIELD - the benefits seem seem so great for the visual interface cost of a SINGLE additional field.
Not saying I'm against any of those other additional custom fields, but until you all iron out if we should be go or no-go for a priority field, or whatever --- it's TAGS. :-) (and no, not in the expanded (body) text field - that's for comments or elaborations, and way too editable to sprinkle important tags all over)
My main contexts in OF (brought over from iGTD (RIP!)) are loosely based on type of effort I apply to some need. I guess if I can't tag, some contexts will be replaced:
-<context> (<sub-contexts>)
-think (plan, define, solve)
-act (simple action, organize, errand, process)
-look (find, read, research, review)
-communicate (email, meeting, phone, question)
-info (idea, project note)
I gave up on wasting actual contexts on "web" or "mac" because it eliminates my ability to specify
what I'm doing on my mac or on the web. I mean, what can't one do on their mac or on the web? Going with a primary context of "person A" or "person B" also blows up any chance (for me) at meaningful granularity. You'd have to use "Person A" at the expense of "phone" or "email", for example.
A simple "tags" field, though, would allow user to label all kinds of different actions as "web" or "person A". I wouldn't spend all day tagging, I would be super-productive®.
I'm casting my imaginary vote for an iGTD-like tags field.
GTD system attempts by me: Daylite, some pre-Frictionless app I forgot, Frictionless, MS Outlook GTD plug-in (don't ask), kGTD, VooDooPad, Hogbay Notebook/Mori, text files, [Finder tagging in Tiger (with spotMeta)], MailTags, and iGTD. Over the last 2-3 years.
Also a big Omni customer: OmniWeb first (though I don't use it much these days), then Outliner, Graffle, and now OF. Love me some Omni.