I know this is officially documented as a breaking change, but I think the result is bad news for all developers trying to learn HTML5 (http://www.asp.net/whitepapers/aspnet4/breaking-changes#0.1__Toc256770154).
When working on a HTML5 site and no page is specified (e.g. www,mysite.com), the default document renders with an empty form action property, e.g.
<form method="post" action="" id="form1">
This immediately fails HTML5 validation for the default document. All other documents where the document file name is included in the URL works okay.
To encourage more widespread use of ASP.NET with new developers, surely this is a move in the wrong direction.
I have posted a workaround for anyone stumbling across this page via a search, but feel a more robust solution is required from the framework :-)