Xcode was a specific achievement in that it is the first release where all of its compilers are LLVM-based (GCC is no longer included). Xcode also includes the " Automatic Reference Counting (ARC) Objective-C language feature (see also wikipedia ). ARC has revolutionized Objective-C programming by automating memory management without the runtime overhead of a garbage collector. I personally defined and drove this feature late in the schedule of iOS5 and Lion. This is notable for the short schedule for the project, the extensive cross-functional work required, and the extensive backwards compatibility issues that had to be addressed (making it a very technically complex problem).