Three things I learned from my trip to South Africa a few years ago:
1. The airport is essentially a homeless shelter in the area before security. Be prepared to have people try and take your bags and approach you. They will not take no for an answer and are extremely pushy.
2. South Africa is the "western world" for the rest of Africa. People from every African country flock to the open borders hoping for a new life. There is no health screening or check... these people enter and exit as freely as they want.
3. Avoid driving directly through Johannesburg at all costs if you can...
The whole experience is a bit of a wake up call, but you will be approached and have your items touched by people you know nothing about.