Deploying for 12 months or more to Iraq, Afghanistan or any other God forsaken country that your incompetent elected leadership decides to turn into a democracy.
Promotion in the Army up to and including the rank of Lieutenant Colonel (O-5) is almost exclusively based on the amount of time you served. Regardless of how competent you are or your performance, you will get promoted at virtually the same rate as your peers. Being in the top 10% (as judged by people in the human resources command, not your actual supervisors) would help you make Lieutenant Colonel approximately 2-3 yrs faster than others over the course of 15 years. Officers are rated at least once per year but they are not rated in relation to their peers, i.e. human resources does not gather any data that says that CPT A is worse/better than CPT B even if CPT A and CPT B had the exact same boss for the exact same time period.
There is no venue for rating your supervisors or peers and you are not able to move laterally within the organization unless your boss allows you too. If you work for a terrible boss, you must continue to work for him/her until he gets moved or you get moved at his discretion, usually in 2-3 years unless you purposely try to get relieved. This is not actually a bad option because you will continue to receive the same wage and have the same rank regardless of the number of jobs that you are found to be “unsuited” for.
There are no real opportunities for personal and professional development and very limited opportunities to attend advanced civil schooling. Additionally since the Army has seen fit to develop some of the worst areas of the country into its largest posts (Ft Hood in Texas, Ft Drum in New York, Ft Benning in Georgia, Ft Bragg in North Carolina, Ft Polk in Louisiana and Ft Irwin in California), it is virtually certain that you and your spouse will spend a vast majority of your time in an area where there are very few jobs which a normal intelligent college graduate spouse would desire.
Anecdotal evidence shows that most of the best officers depart the Army after their initial tour (3-5 years) due to reasons mentioned above. Because the Army doesn't keep track of who their best/worst officers are, there is no hard evidence to support this wide-spread theory. This allows the Army to remain blissfully unaware of errors in the human resources area while maintaining that it has retained the correct quantity of officers, just not necessarily the best quality. This problem continues to fester as the sub-par officers who did not leave after their initial tour are promoted and gain more influence over younger officers.
Another factor in officers leaving at set points in their careers is because the Army’s retirement system is a “defined benefit” which is only received after a minimum of 20 yrs. Though the retirement pay and benefits are relatively good, the all or nothing nature of a defined benefit system in which the individual builds up no monetary retirement benefit until the 7,305 day in the organization encourages personnel to make their decision at earlier points in their career than they would otherwise. In other words, people tend to stay only if they can visualize themselves as part of the organization for another 15-17 years. It also encourages personnel to leave immediately after serving 20 yrs regardless of their future prospects in the organization.
In general, the US Army is a rigid bureaucracy unable/unwilling to make improvements to the manner in which it conducts itself. To be an agent of change you must remain in the US Army for 30 or more years on the off chance you will be promoted to a high enough rank to be given 2-4 years to change policy. It is more likely that even the organization will have the exact same flaws at the time you retire as it did when you joined it.