I'm a union supporter and dont watch much league, but what ive seen of Billy Slater, he has that ingredient that the absolute world greats have, and thats speed of mind. Brian O'Driscoll wasnt the biggest or the fastest but his mind reacted so much quicker than anyone else, he would react quicker to an opportunity than other players, gaining a half second head start, thats all it takes. You can have all the size and speed in the world, but if your rugby intelligence is not quick you will not succeed. Billy sees things ahead of him and calculates quicker what to do. Thats what makes him a great.
One thing nobody mentions about Slater is he caught some dodgy balls. He had great hands other plays would have dropped them at the speed he hit them at.