- It’s lightweight and easy to install (less than 10Mb download and a super quick install)
- It does not require an administrator account to run/debug applications from Visual Studio
- It enables a full web-server feature set – including SSL, URL Rewrite, Media Support, and all other IIS 7.x modules
- It supports and enables the same extensibility model and web.config file settings that IIS 7.x support
- It can be installed side-by-side with the full IIS web server as well as the ASP.NET Development Server (they do not conflict at all)
- It works on Windows XP and higher operating systems – giving you a full IIS 7.x developer feature-set on all OS platforms
IIS Express (like the ASP.NET Development Server) can be quickly launched to run a site from a directory on disk.
Look at scott Guthrie's post for more information.
Note: Beta will be released soon.