Ok, I don't have much time to write a full reply right now (it's 1am in Chicago, I'm dead tired after 1 full day of DAK, and I'm sitting in a hotel hallway because my room doesn't get wireless access), but I HAD TO reply to this one because it's right up my alley.
The simple answer is: Your essays will be stronger if you wrote the POSITIVE reasons on why you want to pursue your new goal and what you can contribute (your understanding of the DoD and defense industry) to the school instead of ripping on the industry. I was surprised myself, as my first drafts of essays were dissing the industry and telling them why I wanted to leave. Then,without a huge conscious effort, the negatives went away with each successive draft and I focused on what I learned about the industry and what I can share with my classmates. My essays got stronger because of it, even though I didn't think it would happen.
So hopefully this answer (from someone who has the exact same background as you and exact same reasons for leaving the industry for an MBA) will help, along with everyone else's good answers.
If you have more questions, please post and I'll reply when I get back home Sunday night.
BTW, the very interesting thing is, I was approached this morning after introducing myself in my DAK section and saying where I worked. This guy really WANTS to work in the defense industry and wants to learn more about it from me. Go figure!