Xcode includes the Instruments analysis tool, iOS Simulator, and the latest Mac OS X and iOS SDKs.
The Xcode interface seamlessly integrates code editing, UI design with Interface Builder, testing, and debugging, all within a single window. The embedded Apple LLVM compiler underlines coding mistakes as you type, and is even smart enough to fix the problems for you automatically.
Xcode in the Mac App Store has been repackaged, and is now distributed as a stand-alone application. This replaces the Install Xcode package, and adds support for delta updates. Xcode includes a new "Downloads" preference pane to install optional components such as command line tools, and previous iOS Simulators.