The .NET Framework itself has not changed with the integration into Windows Server 2003. In fact, the default install looks as if Microsoft just slapped the .NET Framework 1.1 install on top of it. The tangible benefits of Windows Server 2003 include application hosting and infrastructure improvements:
IIS 6 radically improves over IIS 5 in the hosting environment for ASP.NET web applications. The new concepts of application pools and web gardens enhance application stability, scalability, and performance. The new security measures further pull ASP.NET into the leading technology for enterprise web application development.
Integrated UDDI provides a Yellow Pages of web services within secure company boundaries. This organization and publishing can help Department A avoid redeveloping the same services Department B developed last week.
COM+ adds features that align it with .NET. These include application pooling, application recycling, partitioning, and SOAP support.