I agree that the separation of cabins is not much but then, I've been in biz (other carrier) with screaming kids in the same row, or two rows behind (in Y) and they were loud enough to have been next to me!
- At check-in there are separate queues (or you use Biz)
- The seat is is wider (in some of their aircraft) check beforehand
- The seat pitch is longer
- The seat reclines further (check well know seating websites)
- The seat has extra leg support in recline (see their website)
- Food: on an increasing number of flight (especially from SIN) [not yet from JNB] you get 'Book The Cook'. Check on their website as you can choose a meal from an extra large menu, about 6 or 8 choices, or just opt for the choice on the day.
- Luggage is increased (check their website)
For me, all of the above are worth paying extra for. Some some folks, they will not be.