If you thinking about to upgrade to .NET 4.5 you should consider the Application Compatibility in the .NET Framework 4.5 Guide. This give you good overview about what has been changed and what should you pay attention to. This guide describes changes in following areas: Core, Data, Networking, Tools and Resources, ASP.NET, Managed Entity Framework (MEF), Web Applications, Windows Communication Foundation (WCF), Windows Forms, Windows Presentation Foundation (WPF), Windows Workflow Foundation (WF), XML, XSLT.