.NET Framework is contains many new features as compared to .NET Framework 2.0 and 3.0 although it has a minimal number of breaking changes, so all your existing application running on .NET Framework will keep running without any problem.
However some of the major upgrades .NET Framework 3.5 has added includes the following
- Deep Integration of Language Integrated Query and Data awareness
- Added ASP.Net Ajax for creating efficient and flexible controls
- Added new web protocol like AJAX, JSON, REST, POX, RSS, ATOM and several new WS-* standards
- Added Full tooling some new workflow enabled technologies like WF, WCF and WPF
The Microsoft .NET Framework 3.0 (which is also known as WinFX), is the new managed code programming model for Windows.
It combines the power of the .NET Framework 2.0 with some new technologies for building applications which have visually compelling user experiences, seamless communication , and have the ability to support a wide range of business processes and tools.