Expert answer
Students with strong and weak writing skills differ on almost any dimension that you can think of. In terms of motivation, kids who are weaker writers tend to not have a very strong competence about their ability to write, they often find writing distasteful; it’s not something they really like to do. In terms of their knowledge about different genres, they often have incomplete knowledge. They’re not as strategic in terms of carrying out the processes of planning and revising.
Also they have more difficulty with fundamental writing skills — things like handwriting and typing are slower, the often make a lot of spelling miscues and grammar cues in what they write.