I think people are massively underestimating South Africa. I've got friends who thought Japan would beat them. And even more friends who thought Wales would turn them over. I wish I'd taken their bets, because as far as I can see, up until now SA are basically just doing enough to beat the opposition in front of them. That is a sign of a very very good team.
Against Japan they adopted the Ali 'rope-a-dope' tactic of defending solidly for 60 minutes soaking up all the pressure that Japan could muster, and then opening up in the final 20 minutes.
Against Wales they again defended solidly, and took the penalty points when Wales cocked up. Wales just didn't have the guile to get past the SA defence despite being camped on the SA goal line several times.
So the question for me is do SA have an attacking/running game because they haven't really shown it in the last two outings. If they do, and they decide to bring it to the final then England are going to have to work extremely hard to overcome the solid SA defence. If SA don't have an attacking game, then can England maintain their discipline so that they don't give away penalties in kickable positions.