Islands of Adventure had the theme of visiting these imagined worlds brought to life (you know, the upcoming theme of Disney's Hollywood Studios -slash- The Park to be Renamed Later), in comparison to the existing parks (USF and USH) where you were visiting a studio backlot that took you onto the sets of movies. That was the big separation at the time-- Universal Studios Florida took you behind the scenes, deconstructing the movies, allowing you to admire the art of filmmaking; Islands of Adventure brought the stories and their settings to life for you to experience them as a character.
Keep in mind, this was before you visited Diagon Alley and Springfield in Universal Studios Florida, or Pandora in Disney's Animal Kingdom. The concept of IP-driven standalone lands was quite novel.
Plus, it was Universal designing a "theme park," which they had never done before. Previously, they had built production facilities that-- over time, as they learned, and got more ambitious-- began featuring standalone attractions. This was a theme park for the sake of a theme park as part of the transformation of the standalone Florida studio into their first vacation destination resort.