It's more than just one thing, of course. But Spielberg's masterful use of the long take is one of the things that make him so great. While most directors would use the long take as a way to teleport the viewer into another world and show off their skills, Spielberg tries to uses the trick as invisibly as possible. That way, when you're watching the movie, you don't even realise it's happening to you until it's too late.
Watch Tony Zhou explain the Spielberg Oner. One scene, one shot. Pretty fascinating.