These are the most pertinent features to me.
h2. Default compiler
The default compiler for iOS development in Xcode 4.2 is LLVM 3.0. Compared with the GCC compiler that was the default in Xcode 4.0 and the LLVM-GCC compiler in Xcode 4.1, LLVM provides better code generation and optimization than GCC, along with newer language support than LLVM-GCC, including support for ARC in Objective-C and for the new C++ standard, C++0x.
... Looks confusing and is optional.
h2. Automatic Reference Counting
ARC is supported in Xcode 4.2 for Mac OS X v10.6 and v10.7 (64-bit applications) and for iOS 4 and iOS 5. Weak references are not supported in Mac OS X v10.6 and iOS 4.