Lots of wisdom in this TED talk from organizational psychologist (and host of TED's podcast WorkLife) Adam Grant. According to him, here are some key reasons why a team succeeds:

• teams with too many "big stars" are not performing at their best as this means lots of egos which leads to infighting.
• in a team, you can have too much "talent". Make sure you have enough of the key contributors to bring balance and humility to the team.
• value humility as a key success factor and seek to turn narcissists into "humbled narcissists" to combine humility with confidence.