"Most ambassadors may not be household names, but they are crucial to diplomacy, serving as the go-betweens for foreign governments and the U.S. government. Typically, about a third are political appointees. The rest are career diplomats. Their impact varies: Some come and go without making a splash. A few have caused headaches for Washington with culturally insensitive gaffes and missteps. Others are diplomatic rock stars, boosting U.S. influence and prestige abroad, and serving as consiglieres to the president. Foreign leaders take notice when the top U.S. post in their countries sits empty for too long. Here are some of the most conspicuous absences.