There are some new features Microsoft has introduced in ASP.NET 4.0 which consists some enchancements and some of them are real value added for Webforms.
You can set meta tags by using the Page.MetaKeywords and Page.MetaDescription page properties.
More control over view state by using the Control.ViewStateMode property for a page and for individual controls.
Easier ways to work with browser capabilities.
Support for using ASP.NET routing with Web Forms.
More control over generated IDs by using the Control.ClientIDMode roperty.
The ability to persist selected rows in data controls by using the GridView.EnablePersistedSelection and ListView.EnablePersistedSelection properties.
More control over the HTML that is rendered by the FormView and ListView controls.
Filtering support for data source controls by using the new QueryExtender control.
Charting support by using the new ASP.NET Chart control.(I love this really now I can get rid of OWC )