w/o really thinking about it, I was compiling and developing for .NET 4.5. This obviously leaves out everybody on lower versions, so I managed to bunk it down to .NET 2.0.
On info on how it's possible to use extension-methods in .NET 2.0, read http://schoening.it/blog/post/2012/01/26/Solution-Cannot-define-a-new-extension-method-because-the-compiler-required-type-SystemRuntimeCompilerServicesExtensionAttribute-cannot-be-found-Are-you-missing-a-reference-to-SystemCoredll.aspx.
