The patient population tends to be much younger and healthier. The flipside is that they tend to be much more reckless so self destructive behavior like smoking and engaging in risk-taking activities is rampant.
They also tend to be either massive overutilizers or underutilizers of health care. The overutilizers go in for minor aches and pains because there's no co-pay and it will get them out of work or certain aspects of their duties they find undesirable. The underutilizers are the young men and women who try and tough things out or fear recourse if they seek medical care so they tend to avoid docs.
Another huge aspect of military medicine is the career implications you can impose on someone as a doctor. In civilian practice, there's little recourse of giving someone a diagnosis, however; putting certain diagnoses in a servicemembers record can be a career killer. Imagine being in 17 years, 3 years from retirement, then some doc puts "fibromyalgia" in your chart and now all of a sudden you're being looked at for medical separation.