First and foremost, your story.
Whoever sits back to read your essay wants to know you, yes, YOU! A generic essay is very boring to read. People want to read a good story that deliver a message (who doesn’t like good stories?), hopefully about who you are and what experiences you have had that made you unique.
Another bad example is essays that are basically a laundry list of big or small accomplishes in the student’s life, with a little bit of stuff here and there. There is no focus, there is no thesis. The reader glances through and ended up remembering nothing.
Write about things that are relevant to college application and college education. Although some colleges do allow free essays, if you are simply writing about how much you love your mom, it is not going to work for a college essay. The purpose of the essay is to reveal to reader who you are as a person.
A good style of writing is hard to achieve for any high school students. Some essays are just smooth to read, smooth like silk. Your writing should not be mechanic, it should be presented in an interesting and engaging way. When people have hundreds of essays to read, they are dying to read something interesting.
Last but not least, voice.
This is even harder to achieve. Anybody can write, but when you read Charles Dickens or Ernest Hemingway, you know it is Dickens or Hemingway. It is probably because they have a voice. The rich, personal, and distinctive word choice and sentence structure makes it a “sizzling” and enticing piece to read.