The American version cuts out many of the scenes that explain Leon's backstory (as shown to us courtesy of fellow videosift cinephile enoch)...

http://videosift.com/video/leon-the-professional-deleted-scene-leons-past

... as well as scenes that demonstrate how screwed up Matilda is.

The point of Natalie Portman's character was that she was denied any normal childhood and was grasping at her childish perspective of what an adult is based solely on her horrifying experiences in that apartment. Matilda was a little girl who thought she knew (but actually knew nothing) about adult sexuality. Her dress and behavior was the only reaction she could have given the world she saw.

Gone are the scenes featuring Matilda's awkward flirtatious behavior shown as that of a child merely mimicking what adults do. Her story arc in the film was that of a refugee from the absurdities of her behavior and be given a second chance at a normal childhood by the non-sexual supportive love of another. Leon is perfect for the job since he also harbored a stunted sexuality and was in many ways Matilda's equal.

The American version, however, really does suffer because of these cuts. Given this, I forgive Siskel & Ebert for this thumbs-down review.

Waiting for "Matilda: The Professional 2"