> Instead, something magical happens, and the magic doesn't work on Heroku.
To be clear, this is because the 'Welcome page' shows info about the server, so you NEVER want that accidentally leaked to the world at large by accident, so the route gets disabled in production mode.
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.)
To be clear, this is because the 'Welcome page' shows info about the server, so you NEVER want that accidentally leaked to the world at large by accident, so the route gets disabled in production mode.