Rufus King's claim to fame was that he was one of the signers of the United States Constitution. He was also a lawyer and a diplomat, but in signing that document he became part of a very elite group that has a special place in American history. One admirable thing about him was the fact that he was against the expansion of slavery into new territory, and the idea of slavery in general.
Aside from being part of the foundational document of our nation, King was a successful ambassador to Great Britain. He must have been good at his job, because when Thomas Jefferson (a member of the opposite political party) came into office King was allowed to keep his ambassadorship while many others lost their jobs.
King was very intelligent and had a way of speaking that got right to the point of things. He was not an imposing guy physically, but his ideas came across loud and clear thanks to his high level of speaking ability.
There isn't too much to criticize him with or find particularly dislikable. He seems to have done his best in most things and was well respected. It could be said that avoided harsh military service during the American Revolution, having only a brief job as a general's aide, but that's about it. At the time, not joining up quickly enough would have been a sign of either cowardice or misplaced loyalty. King did eventually join toward the end of the war, and none (now-a-days) would doubt his loyalty to the cause, but he just doesn't seem to have had the soldier's heart.
King came from a very "local" piece of New England with very provincial views. Over time, he developed a broad mind and came to have view the United States as a collective rather than a grouping of parts. He may not be the most widely known name in history, but that certainly isn't a comment on his character.