A premise of Agile is to have teams of competent members. Agile only works well of you have good team members. Here are criteria that one should look for in a team member:

1. Will they fulfill their commitments? When they promise to do a task can you trust them? Agile is about meeting commitments, taking responsibility for them. and truthfulness. There is no room on an Agile team for lazy developers. I know we may incorrectly estimate the time it will take to do a task but you will only do this so often if you know you have to meet that commitment in a sprint.
2. Are they avid learners? What was the last book they read in their field? What is the last book they read outside their field? How many seminars have you attended? What are your overall learning goals? It is so easy to stagnate. We all need to be stimulating our minds.
3. Do you work well with others? We work together and may disagree with each other but we are team with a common commitment.