I created a custom content type called "test".
The columns are:
The column "Publisher" is already a column which comes from SharePoint itself.
I changed the default value to "afraIT" and published the content type. I also started the process "Content Type Hub" and "Content Type Subscriber".
The content type is available in my library, but the default value (afraIT) is not set.
I can change the description. That is working, but the default value is still empty.
UPDATE: I found this link: http://sweechingho.blogspot.de/2008/07/sharepoint-site-column-default-value.html
They mention that you have to set the content type as default. Only then, the default value is shown. I tried it and it works.
Is this really the only way Microsoft?