Nobody argues the financial success of the movie, I think they're more concerned with the fact that the movie didn't do anything new really.
As great a movie as it was, it was pretty much cut from the same cloth as the first movie. The story was written so that it would play out exactly like the first - the guys must go from zeros to heroes, Peter must have a romance with Dana, there must be interference from a petty, government official, etc.
Heck, they even went out of their way to have a giant figure walk the streets of New York and Slimer put in an appearance. The only new territory in the movie was the relationship between Louis and Janine.
This made for a blockbuster movie, but it didn't win it any critical acclaim.