Customize access denied page
A custom access denied page can be defined under /admin/config/system/site-information. For example, one common 404 error page is provided by the Search404 module.

All you have to do is define your own page, perhaps with a teaser from the requested page and informing the user that access to the full content requires that they be logged in.

This will probably require tha you put some php code into the page body, or you might use the Search404 module as the basis for developing your own module to handle this.

Edit: The more I think about this the more interesting the question becomes. I think a case might be made for a 'View Teaser' permission in addition to 'View published content'. In other words, the page could be available for all users, but the teaser would display instead of the full content, and a message could be displayed indicating that the user had to be logged in to view the full content, much like is done for comments now. Perhaps an idea for a contributed model (could not find one on D.O.).