As an instructor in both settings, one of the main differences I see in the writing expectations of high school vs. college is inquiry.  As an AP teacher, I used to have students write to specific prompts.  I even trained them in how to write an effective essay quickly.  I remember students’ frustration when they received a prompt they didn’t like; it really impressed on me the importance of letting students choose their own writing topics.  Not only do students enjoy inquiry-based writing more, I enjoy grading it more!