If someone ask's you why you think made John Williams so great. Ask them to hum out any piece of music from almost any movie... 90 chances out of 100, it will be from one of the following: Jaws, Superman, Star Wars, Indiana Jones or Jurassic Park or something derrived from or artistically similar to one of those themes. Even the Star Trek franchise couldn't have survived...or rather revived without John Williams. Okay, I know that John Williams didn't write a note of music for Trek...but think about it, the Motion Picture was supposed to be an answer to...Star Wars, and thusly the music was made to sound similar to the themes of Star Wars.
The music from Jurassic Park almost instantly became one of the most recognizable themes just over 10 years ago from this writing (June 9, 1993 marked the opening of the movie) and 10 years later, fans of the movie still thrill over watching the kids (Ariana Richards & Joseph Mazzello) get chased around the island in the visual medium, and my belief is that the movie wouldn't have been as good without two things: the beautifully played out looks of terror on Ariana's face and of course the background music to pace the entire things.