I seriously need some tips on writing a college essay. Read what happened....(not long, I promise)?

I just got my essay back, and the professor gave me a D !!! I am an excellent writer, and he said he gave me a D because it is too flowy, and too trite. WTF? I seriously believed I was going to get an easy A. I am a freshman in college, and my question is: In college freshman writing, do the professors want very crappy writing, or do they want very direct and snappy writing, that isn't ostentacious? (showy) Anyone have any tips on how to write an effective essay that will get me an A or a B? Remember, I am in college, and they apparently want different writing from that I was taught in high school. Any tips?