.NET 4.5 dramatically optimizes the TPL and other multithreading framework code without using anything new that requires Vista or later, and this is vitally important for the async/await keywords. This is my fourth company in five years and every single one mandates XP as a baseline spec for their products. There's no reason not to support Windows XP; nothing changed in WPF that will require a newer version of DirectX. Nor did anything in the CLR thread pool change to REQUIRE Vista or later. This is basically planned obsolescence.