Based on my experience working here for two years, these are my observations. I accepted employment elsewhere four months ago, so these opinions may or may not still be accurate, but these opinions are derived from what I observed while there.
Largely a group of recent college graduates (or staff with more years of experience but not a great deal of talent, relatively) the group tends to be inexperienced. Management is pretty good at trying to convince staff that pay is competitive and technologies are cutting-edge, but a little research shows that this simply is not the case. Desktop operating systems, programming environments, and other technologies tend to run (literally) several years behind the rest of the world, although the staff generally is not aware of this at first. As employees gain experience and begin to realize these problems, they tend to leave for better-paying positions at other organizations that can offer better pay and a more cutting-edge use of technology.
Additionally, due to policy (such as SOX compliance and other internal policies) the actual task of writing code and promoting it from test to production environments is terribly inefficient. For example, changing a single line of code can require multiple levels of approvals, up to 18 (or more) different forms of documentation (including all testing, and creating artifacts) be completed by the programmer! Since there is no staff available to handle this administrative overhead, the programmer is actually responsible for coordinating, implementing, and documenting all of these steps using multiple, disparate systems. All of this is required, even if the change required affects only a single line of code. And the process can take from several days to weeks. I have worked for other very large organizations with very strict documentation and controlled processes, organizations that are SOX compliant, and never ever have I seen such terrible efficiency.
Another downside is the competency of lower management (also called BPLs or business project leaders.) This group is responsible for handling pretty much all of the day-to-day project and staff management. Sadly, the group is fairly incompetent as they generally have not performed hands-on development or engineering duties in some time, years in some cases, nor are they required to remain current or prove their competency through periodic exams. Also, generally speaking, since the system of promotion is based on seniority rather than competency, the employees who remain here the longest (and thus get promoted) are generally the ones who have had to remain because they have not qualified to leave -- they have not received outside offers -- to work for other higher-paying, much more rewarding careers in other organizations. Those who do receive offers, and several did while I was there, do accept those offers and are quite ready to leave.
Lastly, a large amount of the development and engineering efforts are outsourced (at much higher billing rates) to companies such as Accenture. I will not speculate as to why the USPS continues to spend enormous amounts of money on outside organizations while supposedly trying to make critical, life-saving budget cuts. But I will say that it is very clear to me, after spending time with these programmers, that outsourcing their work leads to resentment and lowers morale.
Again, having worked as a software engineer at other very large organizations, thus being qualified to compare things from an engineer's perspective, I am disappointed with my experience at USPS, at least in the Information Technology area. I hope they do some sincere self-evaluation, but having witnessed their internal operations first-hand, I highly doubt this will ever happen.