Full error:
Could not load type 'System.Web.HttpContextWrapper2' from assembly 'System.Web.Abstractions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.
[TypeLoadException: Could not load type 'System.Web.HttpContextWrapper2' from assembly 'System.Web.Abstractions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35'.] System.Web.Mvc.MvcHandler.ProcessRequest(HttpContext httpContext) +0 System.Web.Mvc.MvcHandler.System.Web.IHttpHandler.ProcessRequest(HttpContext httpContext) +36 System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() +181 System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& completedSynchronously) +75
处理办法：
检查开发用机是否安装了.NET 3.5 SP1，而MVC的web site使用的是MVC Preview 2。下载.NET 3.5 SP1，一切正常了。