#1 for me would be situational awareness. Good tanks know that ninja # 2 is about to pull hate, and react accordingly to prevent it. They see that roamer sneaking up behind them and react accordingly. They know their healer is busy running to dodge an AoE, or rezzing someone else, so they burn a cooldown or two to make life easier.

All the gear and stats in the world can't help you if you're clueless about your surroundings.

Also, since the tank often ends up as the de facto leader of the group, patience, and the ability to explain things to newcomers is always an asset.