Another very good drawing application, especially with a pressure sensitive pad, is autodesk sketchbook pro. It's just for drawing through not modeling.

I don't know of any application where you "draw" 3d shapes. Although google sketchup is pretty darn close. But even then you start with simple extrusions of a starting 2D form.

To me it sounds like digital sculpting is more in line with what you are describing and I would look into mudbox. It's far more intuitive although less feature rich then zbrush.