It blew my mind to enter college and peer review other people's papers. Up until that point I understood that there was a range of capability with writing, but I figured that in college courses the floor would be pushed up. I was dead wrong, and it was shocking to me that these folks had managed to get all the way through high school writing like 8th graders.

ETA: This is over 20 years ago, btw. I hear it's gotten materially worse. That terrifies me.