While the AB's certainly have a good chance of winning, I would say its a substantially lower chance than last WC. Last WC you guys were on a big winning streak (OK you did lose one match against Aus right before but still). Essentially you looked pretty unstoppable and on top of that you were playing at home, which is not a place you generally lose. Over the last 3 years you have lost to SA, and England as well as drawing a couple times against Aus, so it feels like you're beatable. I feel like the other big teams will feel have the belief they can win which could make all the difference.
That said, you are still definitely the favourites, a class outfit and will be very hard to stop, but it's far from certain.
As for SA's chances before last years tour I would have said 2nd favourites but after it... not at all, we're going to have to go through Eng/Aus/Wales -> NZ ->Eng/Ire/Aus in the knock-out stages which is probably about as difficult as it gets. I wouldn't right us off but I wouldn't put any money on us either.
I'd say Ireland are second favourites, due to a relatively easy run (only in comparison to the other pools) and in general being in good form.