So what is the big deal about the new .NET compiler Roslyn? The two major thing that I see are the deep hooks that it opens up and that it is open sourced. The deep hooks are very interesting because they are going to allow tools producers such as Telerik, JetBrains, PostSharp, and CodeSmith hook into the compiler at a deeper level, allowing greater flexibility and less duplication of effort (they don't have to parse anymore). This is a similar move to what Microsoft started back in Visual Studio 2010 when they began breaking the components down into individual parts and providing hooks for extensions. The number of developer accelerator tools that are available has seen a dramatic growth since then. The open sourcing of the compiler is a bit subtler. The Mono Project has been maintaining a compatible compiler for the past ten years, and has helped advance the language. Now all of those people can contribute directly to a shared source tree, reducing the duplicated effort and giving a vast number of coders time to advance the ecosystem in other ways.
This is a big win for anyone who codes in .NET, and it will be years before the ripples of this latest Microsoft move die out.