These are the things I learned in my 20s, mostly during my "quarter life crisis":

1. Don't mortgage tomorrow to pay for today. The 20s are a prime target for credit card companies and lenders, and you will notice it too - during freshman orientation at University, how many times have you seen a table to sign up for your first credit card with the promise of a free Teddy bear? Before you know it you're $10k in debt on top of your student loans, and won't see that paid off for decades on your entry-level salary. Stay out of debt for AS LONG AS POSSIBLE and use it only when NECESSARY TO SURVIVE. Realize that you will end up paying double, maybe more, for that round of drinks at the bar because you put it on credit instead of saving the cash.
2. Greatness will not find you: you have to find it, and work damn hard for it too. All through the eighties and nineties we were taught to follow our dreams, and not settle for less. We were taught to do work for the love of it, not for the money. This is a nice sentiment, but reality comes crashing down on you once you graduate. You might feel slighted, lied to, or unfairly left out of the wonderment that life was supposed to bestow on you. Is it possible to do what you love, be successful, and be happy the rest of your days? Absolutely - but you have to WORK REALLY HARD, EVERY SINGLE DAY to get there. You have to do those 10,000 drawings, play 10,000 gigs, code for 10,000 hours, etc. in order to get there. Figure out who your heroes are in your chosen lifestyle, and focus on the stories of the boring crap they had to do in order to get there. The media loves stories of people who just happened to be in the right place at the right time and got lucky, but for the other 99.99999% - and you - this will not be the case.
3. It's OK if you don't know what you want to do with the rest of your life, but don't use that as an excuse to stagnate. As the great Baz Luhrmann song that was released the year I graduated from high school says: "Don't feel guilty if you don't know what you want to do with your life...the most interesting people I know didn't know at 22 what they wanted to do with their lives, some of the most interesting 40 year olds I know still don't." Your 20s will probably be some of your fondest years; you have the freedom of living on your own, without the responsibility of having a family to care and provide for. Don't waste it building a career you're not interested in just because you're supposed to already have it figured out, and don't waste it stressing because you haven't figured that out yet. And, worst of all, don't be content with sitting at home doing the same thing you did through your teens just because it's comfortable; if you don't want to build a career or go to school, at least keep learning, trying new things, meeting new people. Enhance your experience wherever you can, and see what new paths open up for you.
4. Even the worst job you will ever work at has something to teach you - so stop complaining and pay attention. I have worked at some pretty awful jobs in my life; positions that included slicing my fingers open, being literally screamed at daily, handed a bounced paycheck, and one that after a few months made me literally want to vomit every day. But instead of feeling sorry for myself and just toughing it out until I could find something else, I paid attention to the things I didn't know and how I could learn them. People were making a lot of money, I paid attention to how. Leaders were making strategic decisions, I figured out why. I was proactive and moved up, because I knew that when I left I would have a better title, better resume, and higher pay grade. (Interesting footnote; this makes a great answer during your interview for that dream job - what did you learn while flipping burgers at a grease joint?)

That's all I can think of for now, but I'm sure there's more, because this is a pretty deep question. You can expect a lot of varied responses from anyone in their thirties and beyond.