What should graphic designers ultimately know?

How to sell his/her self and work. I know many good designers that are not making a living from their designs. I know a lot more middle-of-the-road to very poor designers that get all the attention/work. The difference is the latter knows how to "sell" while the former, as good as they may be, don't have that skill. Design is a business, not an art. Want to be the struggling artist? Don't get involved in the business of designing.

Back when graphic design was just writing, Mir Ali of Herat noted: A Calligrapher needs five things a fine temperament understanding of calligraphy a good hand endurance of pain and a perfect set of implements The first and fourth items on the list are useful for dealing w/ clients (and the first is a nice quality in co-workers), and understanding clients and their needs. Extend the second to include graphic design, images, illustration, typography (ideally both micro and macro), grids, &c. Sadly, the perceived need for the third has gone out w/ the prevalence of typing, but really, it’s far faster to hand letter a thumbnail than to fiddle w/ the font menu. The last, well, it was important when one was directly creating the final product, these days one might note instead the knowledge of how to create a press-ready file.

To be able to ask the right questions and infer the correct requirements. Many times, its an internal monolog but a lot of the times, its making the client aware of their real needs and then work towards it