One thing that stands out for me (in addition to his calmness) is how everything he does has purpose. When heading the ball, it's usually a cushioned header to a team mate, or an angled (and targeted) header to keep possession - he doesn't just go for height and distance to clear his lines. Similarly, when under pressure with the ball at his feet, he rarely (if ever) clears the ball up field to no-one in particular - it's always about keeping possession and keeping the ball away from danger.
So often we see defenders that clear the ball randomly only see it come straight back again and invite pressure, but Matip is one of those rare defensive players that is already thinking about where his team mates are when the ball comes to him, and seems to have an awareness of space and timing that is great to watch.