A bit behind the times here but in my defence I’ve been up to my ears in nappies and things :). I’ve just been having a read of the ASP.NET 4 and Visual Studio 2010 Web Development Beta 2 Overview whitepaper and spotted a few things which I don’t think were there in Beta 1 (I’m sure this list isn’t exhaustive but let’s have a stab at it anyway):

• Expanding the range of allowable URLs
• Alter the max path / query string length
• Specify invalid characters used for the valid URL character check
• Request validation logic is customisable
• You can customise HTML, URL and HTTP Header encoding
• Perf monitoring for individual apps in a single worker process
• JSONP support in ASP.NET AJAX allowing you to access services in a different domain
• <asp:chart /> control
• HTML encoded expressions (automatic HTML encoding of markup expressions)
• New project templates include basic styling and membership functionality
• Controls render XHTML 1.0 Strict markup by default
• For controls such as FormView, Login, ChangePassword etc the outer table element they render can be suppressed
• CheckBoxList / RadioButtonList can render as <ul> / <ol>
• <asp:menu> renders as <ul>, keyboard navigation improved, gets ARIA roles / attributes
• More control over HTML generated by Wizard / CreateUserWizard controls

And of course all the other good stuff is still there.

Also check out the breaking changes document. I spotted a minor discrepancy wrt to ClientIDMode between the overview and breaking changes documents. The overview states that controls will default to AutoID whereas the breaking changes doc states that the default setting is Predictable. I did a quick test and it looks to me that the breaking changes document is correct. ie Predictable is the new default.