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.
Love It or Hate It: The 2010 televison adaptation with David Suchet falls under this, having a Darker and Edgier tone dealing with Poirot's moral and religious struggles and paying less attention than either the novel or other versions of the story to The Reveal, mostly on the grounds of It Was His Sled. In this version, Poirot is horrified when he finally deduces what actually happened, and the killers openly contemplate murdering him (and the train manager) too to cover up their crime when he announces it, since they could conceal it easily as they are all stuck on a train in the middle of nowhere in the middle of a snowstorm, and both they and Poirot know it. Both are issues that the original novel skips over but probably crossed the minds of most readers when they actually read it, so whether this version gains or loses points for bringing them up is up to the viewer, though neither the identity nor the outcome are substantially changed (the latter is left slightly more ambiguous, but it still seems that Poirot makes the same decision as in the novel, just with much more reluctance). Like most episodes of the series, some changes are made to certain characters and one is omitted.
Values Dissonance: A mild example, in Poirot's referring in his notes to one of the passengers as an "American subject". Americans find it politically objectionable to be called subjects (to whom?) rather than citizens.
Invoked in the television adaptation when Poirot and a couple of the passengers witness a stoning of an adulteress in Turkey and Poirot excuses it with this trope; it comes back to haunt him later when the girl who saw it and angrily disagreed with him asks him how he could stand back and let that happen but (in this version) is revolted by the vigilante justice of the murderers, despite the Asshole Victim being far more deserving of his fate than the adulteress was. He fails to give himself a satisfactory answer.