These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
The Twist Ending of the film begs the question of Chance's true nature: is he The Fool or The Messiah? The filmmakers intended the former, but knew the latter interpretation was possible.
A rather twisty interpretation suggests that he started as the former, and became the latter as people grew to believe in him.
Arguably, the film's existence was based on this. Kosinski wrote Chance as a representation of his fear of a style-over-substance world, having no idea someone could actually identify with such a character. Though they disagreed on the implications of his rise to power (Sellers saw it as the meek inheriting the earth), Kosinski admitted that Sellers understood Chance better than he did.
Comedy Ghetto: The most likely reason Sellers didn't win the Oscar, especially when a mentally challenged character is usually Oscar Bait.
Fridge Logic: A rare in-movie example. Louise, a black cook who knew Chance his entire life, complains about it.
Louise: It's for sure a white man's world in America. Look here: I raised that boy since he was the size of a piss-ant. And I'll say right now, he never learned to read and write. No, sir. Had no brains at all. Was stuffed with rice pudding between th' ears. Shortchanged by the Lord, and dumb as a jackass. Look at him now! Yes, sir, all you've gotta be is white in America, to get whatever you want. Gobbledy-gook!
Idiot Plot: A rare case where people not asking obvious questions is part of the story's point.
Misaimed Fandom: Kosinski was frustrated that film audiences (especially younger ones) weren't unnerved by the implications of Chance's rise to power — that style will always trump substance; that people can become so warped by TV, etc. that they will not develop their own personality — or even realized them. Of course, Chance is an atypical example of this trope in that he isn't a bad person, but more a victim of circumstance. Due in part to Values Resonance, however, this misaimed fandom is not as much of a problem now.
They Copied It, So It Sucks: Kosinski was accused of plagiarising 1932 Polish novel The Career of Nicodemus Dyzma. Nowadays it's generally assumed the plots are not really that similar. While both feature a sudden rise of an unexpected character, Nicodemus was not simple-minded, he was poor, and at the end of the novel is usually not perceived as sympathetic.
Values Resonance: The growing importance of style over substance, sound bites, etc. in media and politics since The Seventies has granted both novel and film this.