GTD is for getting things done, not tweaking your system
Before you start thinking about configuring your software, whatever it is, I think you should reflect on the point of GTD. I say this because Omnifocus, for example, will take up a lot of your time as you endlessly tweak it, trying to optimize, optimize, optimize. Trust me. Things has a simpler presentation and I have found it much less distracting. And I use it to manage several concurrent business projects: I'm a software project manager. It's not a replacement for project management software (and neither is Omnifocus). GTD is about not having to think. Omnifocus makes you think too much, IMHO.
A feature Omnifocus has that Things does not is the ability to use and present sequential actions in a project. Sounds great, but in reality I've found that most of what seem like sequential things are not really that sequential: they overlap and can be begun in parallel. Also, in real life, projects have multiple dependencies and a single line sequence doesn't capture the work. So you wind up ignoring sequential most of the time.
A feature Things has that Omnifocus doesn't is tagging. After two years of use, this has emerged as a key benefit. Omnifocus lets you categorize an item with two variables: its Project and its Context. But there is at least one more variable for many items: Person. And an item can relate to several people. Things has Projects and Tags. In Things if I have a Phone item for Project X and I need to call Bob and report the results to Bill, I put the item in Project X and tag it with tags Phone, Bill, and Bob. Now, if I run into Bob in the hallway, I search for the tag Bob and I know what to ask him. In Omnifocus the question would be buried in the Phone context and I'd lose the opportunity.