I think there are a lot of qualities needed by a halfback. The ability to read defence, a good passing game, a good communicator, a good kicking game (particularly short kicking, someone else can kick you out of trouble), they need to be a good defender, they need to be tough, they need to be able to read the game and set up plays at least a couple of tackles in advance. They need to be willing to run the ball. The need to be able to adapt to the game and create opportunities. They need to run the side. A dash of speed is very useful...an alert and responding team around them when they make a break is more useful.
How many boxes does Sexton tick? Or...is speed the most important? Hmmm....