Perhaps Spielberg realized that too. New creatures were a treat but the raptor thing has been done to death. Sam Neill and Jeff Goldblum chemistry was the best (neither appeared together after the 1st and both looked lost without the other).
Overall, the first was the best because it was a new experience. The experience has come to an end but with a huge take at the box office, I suppose I will find myself watching JP4 in 3 years.