How to have a difficult conversation. Whether it's asking your co-worker to turn down their music, telling your boss you're quitting or letting an employee go, you're going to have tough conversations over the course of your career. Your life will be much better if you get comfortable with being straightforward. That doesn't mean rude, of course; you can be direct and kind at the same time, but you do need to assert yourself and get comfortable with difficult topics. Speaking of direct ... 

How to stand up for yourself politely and professionally. There may be times when your employer does something that you need to push back on - for example, offering you a promotion with significantly more responsibility but no raise, expecting you to work unreasonable hours for months on end or violating a labor law. In these cases, it's key to know how to professionally advocate for yourself. Usually that means being assertive but not aggressive, calmly explaining the issue and being direct about what you need. For example: "I'm happy to pitch in when needed, but this schedule has me working seven days a week for the next month with only two days off. I'm not able to do that because of commitments outside of work, so let's talk about how else we can structure this." 

What you're good at and what you're not so good at. Early in your career, it's pretty normal not to have a well-refined sense of where you shine and where you don't. But if you've been working for most of your 20s, by the end of them you should have fairly nuanced information about what you're better at than others, what you're much better at than others, what you want to work on improving in and what you should probably avoid altogether. 

What to do when you make a mistake. At some point, you're going to make a mistake at work because you're human. When you do, how you handle it will often matter more than the mistake itself. The key is to take responsibility for what happened; don't make excuses or be defensive. Let your boss know what happened and - this is crucial - how you plan to ensure it doesn't happen again. If you do that, you'll have proactively addressed what your manager probably cares about most and he or she is less likely to impress the seriousness of the mistake on you. 

Your reputation matters. Your reputation for doing great work and being easy to work with is what will give you more and more professional options over time. It's what will let you avoid bad jobs and bad bosses and what will give you a safety net when you need to leave a job quickly or find a new one across the country. That means that it's not worth doing things like leaving a job without notice or telling off your boss, and it's worth it to go above and beyond to build a reputation for excelling.