I talked to a bunch of FSOs at an embassy about this. What an odd system. Basically, you can work your ass off as a State Department guy for twenty years or so and then, "Congratulations! You're the new Ambassador to Nigeria (or somewhere else that really sucks)!" while some jackass who has never worked a day in his life as a diplomat gets to be the Ambassador to Germany (or somewhere else really cool).
That would be like having a CO of your squadron just show up out of thin air from civvy street and you have to work for him as a DH.
Like I said, weird.
Well, not exactly. The political appointees do go to the cushy ambassadorships like Germany and the UK. That is because those posting do not need the fine touch or detailed experience in foreign policy to keep our relations on track or mind our nations interests in those countries. It is all on auto pilot. Unfortunately for the career diplomats, they get the crappy ambassadorships just because they have the background to deal with the thugs, thieves, malcontents, and nut cases that are found in the back waters of this planet. As a tax payer, it sounds like a pretty good use of manpower. The talent is going where it is needed.
For fun, consider the CINCs from CINCUSNAVEUR (based in London two blocks from the US embassy until a few years ago) and who has been appointed the Ambassador to the UK.
Another fun fact is that FSOs must resign their commissions when taking a post in Taiwan. Since we officially recognize the Chicoms they will not tolerate us having diplomats in Taiwan. When their tour is over, the FSO is recommissioned without a break in service.