Jonathan Rentzsch has an interesting blog posting listing the top 10 things he loves about Objective C and the top 15 things that he hates about Objective C. If you are at all interested in Objective C, check it out.
I would have to say that categories is my favorite Objective C feature. In case you don’t know, categories allow you to add methods to a class definition without changing the original class. This means that you can add functionality to classes that you don’t even have the code for. In the past, to accomplish something like this, I would create a subclass of the original and add my method there. Categories are much more useful because any object of the type that is being categorized automatically obtains the new functionality without any changes in code.