iOS 4 has tons upon tons of awesome new frameworks and APIs but nothing (IMHO) beats the addition of Blocks to iOS.
Blocks is Objective C's take on closures, which are commonly used in JavaScript AJAX code as callbacks for processing the response to a network request. For example, when using jQuery.ajax(), you'd often specify the callback that processes the AJAX response as a closure that's in-lined with the rest of the code that dispatches the network request. The benefit of this syntax is that your request and response code is kept close to each other and can very conveniently access the same set of data and variables.
