People who were paying attention during undergrad should know this; I realized early on that a lot of your experience had to do more with the people involved than the material. A good advisor is no exception, and from what I hear, is even more important in grad school.

It's also pretty true outside the ivory tower. Surrounding yourself eight hours a day with people who amaze you with their skills and/or having a de-facto mentor can do wonders. A lack thereof can leave you feeling demotivated, like you are just treading water and not really advancing yourself.