Things has that magic combo that only the really classic applications tend to have: a simple data model that can be used in creative ways to do powerful things and a carefully polished user interface, with nothing extraneous and features you want in the right places. I really like it.

OmniFocus is the exact opposite: a data model with baked-in structure and not much potential for variation, and a user interface that needs a lot of work. I don't dislike OmniFocus really, and I'd suggest to everyone to try it, but Things is adorable.