Depends on how far you intend to get into it. At the base a librarian doesn't need that much more than the skills of somoene working in retail.
But if you're ever going to manage a library at anything beyond just following a template, then you need to understand different forms of information storage and retrieval.
If you're going to manage collections then you need to keep up with the principles underlying that, and systems of analyzing usage, planning for new acquisitions, and weeding unused parts of the collection.
There's a role for community outreach and both pure marketing as well as involving people through direct involvement in the surrounding area.
Considering how much stuff is moving to digial formats you have to at least know a little something about technology and about the legal issues and security issues related to digital media/commons.
Basically modern librarianship beyond the most surface level is sort of a hodgepodge of a lot of different disciplines, and library schools try to create people who can know at least a decent amount in all of them so that libraries don't have to go hiring people from all over.