South Park is probably not suitable for an 11yo, unless they are very mature - that was pretty much the point of the South Park movie, come to think of it, The fact that it's a cartoon doesn't mean that it's for kids. There's lot's of potentially offensive stuff, though generally speaking, it's taking the micky out of things like racism, as opposed to actually being racist. It takes a very puerile approach to things, but does have some mature reasoning going on behind that.
Try watching "South Park: Bigger, Longer, and Uncut" if you haven't already, and have a think about it - it's very self-referential, and addresses, in their way, some of the issues that people have with it.
With regard to soap operas, I'd say it depends, once again, on DS - there are adult themes, but they are generally dealt with in a responsible manner. WRT language and that sort of thing, your family culture & environment is likely to have much more effect - and school, of course.
I suppose the question with soaps is twofold:
Is DS mature enough to cope with these themes being raised in the first place? and
Do you have the sort of family where you can discuss issue if the kids (or parents, for that matter) have questions?
I'd tend to go the other way round to you personally, but YMMV.