The idea that I was asking too much from them in their writing. This applied to paragraphs, sentences in workbooks and any thing which required a written response in sentence form. When they went to a group school, they quickly learned they could write and I wasn't unreasonable in my expectations.
Poor time management. When they had all day, they took all day.
Grades. They had no concept of A,B,C etc. They also didn't understand number grades either.
Organizational skills. They didn't realize that in order to have the math book at school it needed to be put where they would take it to school. Mine were especially bad with homework. They also didn't realize that bringing the homework to school wasn't enough. It actually needed to be turned in.
Mastering these skills is very important in other aspects of life. Having, listed all the drawbacks, I am VERY GLAD that for the majority of their schooling, my children have been home schooled.