Good things I've learned:
1.) Learning how to recognise good people skills/how to have good people skills - sort of invaluable both in working with other people... as well as just generally a person. I think I'd be a lot more socially awkward if I'd just stayed at home not working an office job and pursued the family buisness, tbh. But that's just me personally.
2.) Learning time management skills - it makes your job easier and it makes your life easier.
3.) Learning by doing things - so many times when I first started I was given work to do that... I really knew next to nothing about what I was working on when I started but I figured out what to do as I went along doing things more and more. Obviously there's only so much you can "learn by doing" just on your own and there will be times you need to make sure you're doing things right.
Bad things I've learned:
1.) The "Peter Principal" is real: people get promoted beyond their level of competence fucking constantly ("Petering out") and it leads to the common workplace culture of shit managers/executives that are in over their heads.
2.) Some industries/certain sectors of businesses have a "boys club" mentality that feels like it's straight from the 50s or 60s. I'm mostly thinking about my business trips with sales people where I work. Casual racism, casual sexism, people acting like they're in the cast of Mad Men... it's just weird & I know of at least one female coworker in the division I work in that felt incredibly uncomfortable on one trip she went on with them. And having been on several of these trips... I fully understand why.
3.) Exploiting workers in high pressure/competitive fields - managers do this all the time because they view newer workers as somewhat disposable. It leads to overworking people who are new, which leads to work-life imbalances that lead to people generally being unhappy and high turnaround with constant faces coming and going. Thankfully this is more of an issue at my last job than where I currently work... but I remember it being something I hated. You feel constantly overworked, like you aren't really valued, and it feels like an unstable work environment.