• Numerous API changes.
• A new hosting API for native applications wishing to host an instance of the .NET runtime. The new API gives a fine grain control on the behavior of the runtime with regards to multithreading, memory allocation, assembly loading and more (detailed reference). It was initially developed to efficiently host the runtime in Microsoft SQL Server, which implements its own scheduler and memory manager.
• Full 64-bit support for both the x64 and the IA64 hardware platforms.
• Language support for generics built directly into the .NET CLR.
• Many additional and improved ASP.NET web controls.
• New data controls with declarative data binding.
• New personalization features for ASP.NET, such as support for themes, skins and webparts.
• .NET Micro Framework - a version of the .NET Framework related to the Smart Personal Objects Technology initiative.
• Partial classes
• Anonymous methods
• Data Tables

Source: Wikipedia