My mentor to me when I joined the job fresh out of college (in a somewhat dramatic tone, which is why I remember it so vividly):
"Gone are the days when you wrote programs with a small number of big functions, and lots of comments. Write code which is easy to read by humans - small functions which do 1 thing and are named after the 1 thing it does."

TL,DR: well named modular code.

Was watching a lecture by Uncle Bob on SOLID and reminded me of this...

Being one of the top devs (and a good student admired by most lecturers) at college, my most humbling experience was when I joined my first job. I thought I knew SQL, I thought I knew C#. I realized in the first week, the thing I didn't know was "I don't know jack".
Thanks to a couple of great mentors (it took a few of them to bring me up to speed :P), I learned that the more I learn something, the more I will realize how much more there is to learn. I used tools to create storyboard animations in WPF, and my mentor would write it all in XAML! I'd write messy SQL and the other mentor just reduces it to a couple of elegant lines. They were like tech gods to my college self, all while being humble and friendly.
They also imbibed in me a sense of responsibility to carry on the culture of mentoring my juniors, which taught me much more than just the technical side of our profession.