Well, I'll take a stab at it why 2nd movies(sequel) are not as good. People want to see something new in the 2nd movie and keeping an audience guessing is what movie goers come to see.Unfortunately, if you go to far to leftfield people end up not liking the movie.It's a delicate balancing act. The first movie gets the audience familiar with the characters and the 2nd movie needs to develp the character but also have a plot that is enjoyable to watch.
You know there's an easy formula for success it's called T & A.
Provide enough of that and at least the males will enjoy the movie.
How many movies did Marilyn Monroe make and were good story plots? Jayne Mansfield? I always enjoyed Some Like It Hot but many of the movies these pinups played were forgetable. I think Scarlett Johansson is today's modern version of the T & A but she is a good actress.She isn't Meryl Streep b ut she is young and could develo. Then there are the flavor of the month types like Megan Fox and Paris Hilton minus the T part. Pretty women who just are not very good.could be because of the roles they play but I am betting they couldn't do more serious roles since they haven't either developed acting chops or don't seem to put the work in it takes to be a s erious actress.
What did you think of the Die Hard series? I think to a point provide enough action,explosions,car chase scenes audiences seem to eat those movies up resulting in big box office appeal. Sure you need a plot but the audience pretty much knows there is a bad guy and good guy and the bad guy usually loses.