I took some notes on one of the video explaining why the Omni Focus application design is so nice. I liked the analysis, but I actually gave up on the product itself. I now use Thing. However, these are the interesting design points: Application's feature is "shrink to fit" -- The feature set is not overwhelming for the user. The user see what is normally usable for him. The user does not have to jump thru hoops just to get started. The features can expose themselves as needed.
The UI tightens the user's field of vision -- the "focus" feature of Omni Focus
The application has *invisible* scaffolding -- The application provide a structure for the user to do GTD time management, but does not get in the way.