One of my friends serves on the admissions committee at my school. He has told me the worst possible essays are:
-The road less traveled (it's been traveled)
-A poem or recipe, e.g., half pound of intelligence (they want you to be creative but not in that sense)
-Writing your own obituary
-Starting it w/ "ladies and gentlemen of the jury"
-Saying well I want to be a lawyer b/c my grandfather was a lawyer, and so is my father. (This gets you NO points)
-I want to go to law school b/c the guys on Law & Order look so cool doing it.
I don't think that it's wrong to write about wanting to be a lawyer, e.g., one of my friends wrote about her personal background and how it fits into her goal of becomming a criminal prosecutor. However, keep in mind that if they are admitting you, they already believe that you have the drive and dedication to make it. Focus more on who you are, something that isn't obvious from your app. Think about this... what is your passion. Is it family, friends, football, making furniture. In other words, if one of your close friends was going to introduce you to someone, what would they say about you, e.g., "This is John. He's almost finished at Fabulous University. He is also training for the New York Marathon."