You're right, of course, but it's not just that. In previous versions of Rails, the default page (a static file served out of the public/ directory) worked just fine on Heroku. It was only the magical “About your application’s environment” link that didn't work. In Rails 4.0, the static file is gone, and as far as I can tell the entire default page is magic, so on Heroku it breaks in its entirety, too.

It's not a big deal of course, as in any real app you'll quickly define a root route and be done with it. (I do consider it a feature, too, as I really like not having to remove the static file by hand.)