I have never tried Omnifocus (as I do not have a Mac) but I have gotten the impression that they have the same split approach as Nirvana does. Now, where is the simplicity in that? I think it is overcomplicated even to begin with, and even more so when you consider that you then have to find your own workarounds for the very common and real case that you have some tasks, more than one, but not all, that you need to see on your Next and Waiting lists.
Omnifocus allows complete outlining within projects. Both projects and groupings within projects can be made sequential or parallel. It's a powerful, easily misused feature, but OmniGroup does it right, IMHO.