Like a car . No, not one thing, but everything working together . . . speed and control. I believe speed is the essence of performance. Entry rocker is the cornerstone to every shape. With a good entry, your board will paddle faster and easier, like a much thicker and wider board, which is a key element to today's thinner, high-performance surfboards.
Bottom contour is especially important.
Too much concave can cause problems. I like the rails soft from the middle towards the nose, but not cut too deep or bulb-like. Rails that are too round and soft cause the board to be slow and plowy and can cause the nose to dive. Rails that are too hard will cause abrupt acceleration or bite. Outline must always compliment rocker design, and varies from board to board.