For the record, and it may have been improved since your last use of OF, you can nest projects. The only thing lacking is project information on the sub projects. They are under the control of their parent, which may not be what you’re looking for.

I believe you could make the connection in OF folders to areas in Things, and for People in OF, I use contexts. The only important feature lacking in OF, of those you’ve noted, is tags. But with its powerful search capability, that really hasn’t been a problem for me. Global search would be an improvement that developers have indicated will be included in the next (1.7) update.

If you want to read about my early experiences with GTD you can view them at http://johnkendrick.wordpress.com/how-to-gtd/

But since I started to follow GTD more I had to switch to OmniFocus. The reason is that Things lack sequential projects. Without them all contexts are useless. And working strict GTD style is simply impossible. You can’t filter only next actions for given context (tag). And 90% of my projects are sequential so until Thing would implement this feature i’ll stick with OF.

And I have to say that OF is a great app. You’ve got to get used to interface and workflow but then working with it very easy and intuitive.

There actually were plans for a Ruby on Rails based web application for OmniFocus but I think that got put on hold for the iPhone application. Someone has put together a UI for it at RubyForge: http://rubyforge.org/projects/omnifocus-ui/