Efficiency

After intelligence, curiosity and self-motivation (the obvious things), efficiency is the most important trait a grad student can possess.  For many grad students, this is a tough one to learn.  After all, graduate school tends to attract the perfectionist types -- the people who worked hard in high school and college and yet actually still want more formal education.  But for PhD students, there often isn't enough time to do everything perfectly and still have a life (or at least time to sleep occasionally).  At some point extra effort becomes subject to the Law of Diminishing Returns.

Source: relationship-economy.com.  Note that I Google-Image-Searched instead of making my own graph...

The trick is to learn how much effort is required to successfully complete any given task.  For example, getting a 90% on that Endocrinology exam will take much less effort than studying all night to get a 100% (and will have exactly the same effect on your career).  Thoughtfully skimming the journal club paper before the discussion is often just as good as reading it carefully and thoroughly (unless you're presenting!).  TAing for a class you've taught before will take less outside prep time than taking on a new assignment.  Although these examples of efficiency might sound suspiciously like cutting corners, the end result is that you have more time and energy to devote to your own research, which is the ultimate goal of any good grad student.  When you learn how to fulfill your obligations efficiently, you will have more time for the important things.  And this is a skill that will serve you well long after you get those extra letters tacked onto your name.