Grand Central Dispatch
You should do a screencast about Grand Central Dispatch since it was introduced into iOS/OSX in August 2010. It's definitely a lot powerful and useful and a bit hard to understand so It would be amazing you get to us a screencast about that.
I think this is sufficiently covered, as I’ve used it in a bunch of screencasts. If there’s anything more advanced you’d like to see covered, let me know.
Matt Chanda commented
Perhaps contrast it with NSOperation