Nearly all portrayals of schizophrenia in Hollywood film are actually portrayals of Dissociative Identity Disorder i.e. Multiple Personality Disorder
It seems it that Hollywood does get it right when they focus on real life characters like John Nash and Nathaniel Ayers in A Beautiful Mind and The Soloist, respectively. These two films more accurately portray the paranoia, delusions and hallucinations that are characteristic of schizophrenia
Autistic characters in movies almost always displays savant-like behaviors, despite the reality that 90% of autistic individuals do not have savant talents. Movies like Mercury Rising, Rain Man and Boy Who Could Fly fall into this category, seemingly trying to portray the disability as not so bad because of a counter-balancing super-power-like ability.
Unfortunately, Hollywood hasn't given us honest look at autism yet, but at least HBO has tackled the subject with Temple Grandin played by Claire Danes. The movie accurately portrays the real-life struggles of Dr. Temple Grandin, an intelligent and incredibly accomplished, but not-savant individual with autism
Obsessive Compulsive Disorder
A common issue with Hollywood's depiction of mental illness is the combination of multiple disorders in one. For example, depictions of obsessive compulsive disorders are almost always combined with germ or other phobias. In fact, most films never begin to identify the defining characteristics of obsessive compulsive disorders- repetitive processes that the sufferer uses to quell the anxiety caused by this disorder.
Subsequently, we tend to others "OCD" if they are organized or tend to keep things clean, when in reality, OCD would more aptly fit an individual who checked the front door 10 times before going to bed. Exception movie is As Good As It Gets
Perhaps the only mental illness that seems to be presented respectfully is depression. This may be simply because there is little humorous or entertaining about clinical depression.
Since Robert Redford's directorial debut, Ordinary People. Hollywood has seemed to deal with the subject of depression respectfully if briefly. Some of them are The Hours and The Assassination of Richard Nixon