What are the most difficult and useful things people have to learn in their 20s？
There are always going to be people who are smarter, better looking, more sociable, and just all around "better" than you. In fact, you'll learn there are LOTS of people who fit that description. To be happy, then, you have to learn to accept yourself and your shortcomings.
Idealism aside, money = opportunity and therefore matters. Relationships and power matter more.
Friendship isn't forever. Social and economic status does create divides and strains in even the oldest relationships.
Adult life is about learning to live with ambiguity. Remember when you thought you'd have it all sorted out by 30? No, whether it be your career, your relationships or your beliefs, you will always question whether you are making the right choices and will have to deal with regret.