The Base Class Library
Obviously, having a runtime in place is an essential ingredient to the success of the .NET Framework. But far more important to application developers is the availability of a rich BCL that gives access to a wealth of functionality for applications to leverage. In that lies the success factor of various platforms: Windows with its Win32 API, Office with automation libraries, C++ with the Standard Template Library, and so on.
Note: The Multilanguage Advantage
It can’t be stressed enough that the cross-language nature of the .NET Framework uniquely positions it in this picture. CLS-compliant libraries need to be written only once to be directly usable from a rich set of .NET-enabled languages. This provides a huge advantage ...