Pressure. Until you work here, you will not understand the pressure and the stress these people face.
Salary. Because the USPS structured a two-tier pay system a few years back, existing full time employees receive much higher pay than newcomers. This creates a huge disparity that is awkward for both older and newer employees. I won't say its unfair because I knew it when I signed on, but frankly it is why I expect to leave soon. Experienced workers obviously deserve more money as they improve, but there should not be totally separate pay scales. Some newcomers are being offered less than the minimum wage in this state. You can't expect a workforce where one element is reasonably well paid and another is at poverty wages to function without resentment becoming a serious problem.
Finally, I think upper management is failing to properly incent the people on staff. I know many people who refuse promotion because the experience of dealing with upper management is so frustrating and the salary isn't worth the headaches. The result is the people who do apply for promotion are often not the best. And this plays into morale in a way that hurts the USPS. Every employee I know will always do their job and provide great customer service. We love our customers. But many do not promote post office services, even when they might have a bonus at stake, because they so resent the poor management from the top. In one office I know, the entire crew were so angry they refused to volunteer for the national food drive that benefits local food pantries. That's hostility from years of mismanagement