Yoda may have it the homerun with his response.....my first thought was competence....but maybe humility ranks way up there too. The caddie is in control of a fine piece of machinery...yet he must operate in the background and not attempt to take the spotlight off his star. This is not a typical relationship where compromise is the key...but there has to be communication and the caddie must know how to keep his player's confidence high especially during adversity.
We all know what happen to Tiger's first caddie when he tried to share in the spotlight.
HG,
Good answer. The best possible situation for a caddie to be in is one where the player makes the final decision and knows it. Then the caddie is free to speak his mind (being very careful to be positive and not create any negative thoughts or pictures) and know that the player will own the choice and commit to the shot. Whatever info the caddie has is useless if the player cannot handle input. Some players pull clubs by feel and don't want anything more that a "yes." The caddie can still influence the play by how he gives the yardage. He might mention a backstop (slope behind the hole) if he wants to err long rather than short (infinite options here).
One of the rules I came up with early on: "If the player looks like he knows what he is doing, stay out of his way." The caddie must sense when the player needs help and be ready with good advice. I will sometimes offer two suggestions: "It's a five iron up or a six iron down." It forces the player to make a choice.
Humility. The caddie must realize that all his success lies in the player's hands. If the player does not perform, there is no prize. Caddies are sometimes in a teaching or checkrein position. Inexperienced players need the caddie to encourage smart play. Some players need a firm hand to help them avoid going for broke when the situation does not call for it. At the end of a tournament, the caddie can save a player from making a critical error.
In the end, the definition of a good caddie is very subjective. Sometimes he must step up and sometimes step back. He must always realize that it is the player who hits the shots.