Extensibility
Visual Studio is extensible by nature, ultimately consisting of a core "shell" that implements all commands, windows, editors, project types, languages, and other features through dynamically loadable modules called "packages". Microsoft encourages and fosters third-party partners to create modules for Visual Studio via the free VSIP program. However, according to Dan Fernandez, Microsoft "made a business decision to not allow 3rd party extensibility in Express".
Read more about this topic: Microsoft Visual Studio Express