I just want to say before hand that I'm not a Marine, not even in any branch of the military at the moment (leave in 4 days), but I think I do a pretty good job of writing essays like this, so If you're still working on it I'd like to give a few pieces of advice.
Try to be frank with the people you are writing to, think about what they read each day from all of the other people. Everyone has dreams of flying missions, dropping bombs, serving their country. It's great to let the Marine corp know you have these dreams, but I wouldn't spend much time there, instead focus more on what some of the above posters said. Talk about why you came to the decision you came to.
Very much of the application process, for anything, and I'd probably say most of Basic Training is to make sure you are the right cut of person for the job and to make sure your reasons for doing this are correct. Put the reader of the essay in your shoes, tell them what you were thinking about when you decided this was your decision, tell them what some of your short/long term goals are.
If it's any help some of the things I talked about in my essay for my Navy application were: Starting a tradition in my family, to be the first pilot of many and have something to pass down to my children and grandchildren. To have the stories to tell once I retire and the life experiences only the military can give. A steady job in the world of Enron's and Worldcom's who drop employees without a second thought.
Again I'm not a Marine, and your essay was already taken down when I read this, but what I gathered from the quotes was enough to make me think we're both writing about a similar topic. Hope this helps some.
-Rob
