Lesson 1, according the first chapter of Why I Left Goldman Sachs ("I Don't Know, But I'll Find Out"): the difference between a sandwich and a salad.
You needed to be very entrepreneurial and creative. Adding value as an intern often began with getting coffee for the desk every day; frequently, interns also did breakfast and lunch runs. You would literally take a pen and pad and go around to the ten or fifteen people on the desk and take everyone's order. It's a strange concept, but Wall Street looks at attention to detail as an indicator of how people are going to do in their job. If a kid keeps messing up the lunch order, he's probably going to mess up something else down the line.
I remember one managing director-- a few years after I'd started working at the firm-- who was very sensitive about his lunch orders. He didn't eat onion or certain other things. One day he asked an intern for a cheddar cheese sandwich, and the kid came back with a cheddar cheese salad. The kid handed it to him so proudly: "Here's your cheddar cheese salad." I was sitting next to the MD, so I remember the incident well. He opened the container, looked at the salad, looked up at the kid, closed the container, and threw it in the trash. It was a bit harsh, but it was also a teaching moment. The managing director joked about it with the kid afterward; he didn't make a big deal about it. The lesson was learned.
Earlier: What Else Does Goldman Sachs Have In Store For Greg Smith?; Goldman Sachs Unimpressed By Sophomoric Writing Efforts Of Former Employee;Resignation Letter Reveals Goldman Sachs Is In The Business Of Making Money, Hires People Who Don’t Know How To Tie Their Shoes;Jewish Ping-Pong Tournament Participant / Sixth-Year Goldman Sachs Vice President Is Looking For His Next Challenge;Goldman Sachs Accuser Greg Smith (Might Have) Lied About That Which He Holds Most Sacred