I can't seem to get any ASP.NET sites to run under IIS8 on Windows 8. Under IIS Express they are fine, but I've:
ASP.Net applications come pre-wired with a handlers section in the web.config.
By default, this is set to readonly within feature delegation within IIS.
Take a look in IIS Manager, under the Server name → Feature delegation. If Handler Mappings is set to readonly and your site web.configs have a handlers section, flip the value to read/write and see what happens.
Worked for me when moving to Windows 8 / IIS 8 / ASP.Net MVC 4.