I get the following exception after installing ASP.NET MVC 4 Beta on a machine with ASP.NET MVC 3.
System.InvalidCastException was unhandled by user c
For the sake of completeness...
You can also just turn off webpages altogether if you aren't using them.
(I'm still not entirely clear on the relationship between razor views and webpages, but it seems my MVC app with razor views still works with webpages disabled.)