The fact that Adidas did not produce a new design for the 1974 FIFA World Cup is a testament of how successful the initial Telstar balls became. Adidas even kept the Telstar name, only the text on the balls were updated. The previous balls became heavier when wet, which made them much more difficult to control — players had to kick them harder to reach the same distance. The new balls were coated by a stronger Durlast layer, though, in order to prevent water coming into the ball through the seams. This changed the picture of the games significantly.
Content courtesy of Worldcupballs.info