Experience. I have been part of an FS family for nearly a decade. It is a dfficult environment in which to work, for many reasons. Unlike happycamper, who has never been in an embassy, I can tell you from experience that in my opinion the overwhelming portion of FS people are fairly unimpressive. As I mentioned, I have met amazing people, FSOs and specialists who will be my friends for the rest of my life. But, the majority of individuals were very difficult, rude, demeaning, self-impressed, and self-important.
Part of the problem is that the people with whom you work are not only your co-workers; they are the people with whom you socialize. Especially in impoverished or unsafe countries, often times the Marine House is the hub of American social activity. In any workplace, problems are bound to arise if you socialize and work with the same group of individuals week after week. And, certainly in a place like Paris or Sydney it is easy to socialize outside of the embassy or small expat community. But, there are many posts where this is not feasible. Your co-workers will also determine much about your personal life: where you live, what your security restrictions are, if your spouse gets a family-member job at the embassy, etc... You can imagine all the resentment and infighting this can create. In addition, many of your FS co-workers will be isolated and lonely. The single ones do not have any built-in support networks, and even the ones traveling with family are cut off from their wider social support networks. And just when you start making friends, either they move to a different post or you move on. That loneliness and isolation does terrible things to people. Finally, if you have a spouse who wants to work, this lifestyle will be incompatible with either your career, your marriage, or your spouse's goals.
I cannot say for certain that I would not have traveled the world with the FS if I had to do it all over again. I would, however, create a better exit strategy for myself and for my family. If you are interested in the FS, I would encourage you to try it before you get married and start a family. But, I think you should also plan very carefully to have an exit strategy, in case you don't like it. For example, avoid a two-year, visa mill consular assignment when you begin. Consular assignments in visa mills won't do much for your resume, in case you want to leave the FS. If the FS is your dream, you should certainly give it a shot. You might find that you like it.