The most important lesson that I've learned post-graduate school is that despite the stereotype about librarians and cats, we are really a more dog-oriented-people. Other than that, I've learned that I didn't really understand how pervasive marketing is to the field.
Given the state of the current job market, there is a lot of focus placed on job hunting skills. I can't remember how many times in graduate school I received advice about learning to market myself as a product. While I try to regularly update my LinkedIn, maintain a professional website, and network with others, I find myself having a more difficult time understanding marketing in the broadest sense of the term at work.
It's a difficult transition to make. I know my professional background better than anyone, but there's still a lot to learn about the company for which I work. This might be crossing into advice territory, but I definitely wish that I had spent more time in grad school learning to market the amazing work of libraries and perhaps just a little less time learning how to sell myself as a potential job applicant.
Fortunately I think there's plenty of learning resources out there, which brings me to the third and last crucial thing that I've learned. As librarians, as well as members of our constantly changing world, we are never done learning. Here's more inadvertent advice: don't graduate and expect to let your books, eReader, or what have you, gather dust.