What makes a good keeper? A short question with a long answer:
1. Shot stopping: A good keeper must have excellent reactions, sure hands, good leaping ability, and quick feet. Buffon is the best.
2. Command of the penalty area: A good keeper will catch any and all balls flighted in the penalty area. Few can match Petr Cech on this.
3. Communication: As the goalkeeper stands behind the rest of the team, he has the best position on the pitch to watch the action. A good keeper will communicate with his team mates to properly deploy the defense as well as coordinate passing for the attack.
4. Distribution: The ball often ends up with the keeper and thus he is responsible for properly playing the ball out to the rest of the squad. A good keeper will be able to accurately distribute the ball to his team mates to spark the attack and keep possesion. Edwin Van der Sar and Peter Schmiechel are the best I've ever seen at this.
5. Mental strength: Keepers mostly stand around and do nothing but are called upon to make crucial actions which decide the outcome of the game. A good keeper must have excellent concentration and also be able to bounce back quickly from a goal being scored on him.
6. Positioning: A good keeper will always be in the best position to stop a shot, whereever the ball is on the field. Most good keepers rarely leave the penalty area, as a goal can be scored over the head even from half-field or beyond. Also, a keeper must a good grasp of where his goal is behind him so as to deploy himself at the proper angle to stop the shot. If you want to see perfect positioning, watch Iker Cassilas.
7. Foot skills: An often underrated are for keepers, but this is important. Defenders often play balls back to the keeper and there are times in a match when a keeper is required to be the eleventh field player. A keeper with good footskills brings an added dimension to a team.