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.
Fan Dumb: Some of the complaints about the 2010 David Suchet included that Poirot was being portayed as a Catholic fundamentalist because of his praying with Rosary Beads before going to bed and his much harsher attitude towards the killers; some even claimed that he isn't even supposed to be Catholic. In fact he is indeed shown to be Catholic both in the novels and in other episodes of the series (though not this particular novel, mind). His manner of praying with Rosary Beads— while hardly a sign of religious fundamentalism in the first place- was correct for the rites of pre-Vatican II, which is when the story was set, and his attitude towards the murder and in general can be chalked up to just his sensibilities. It's not the first time he's refused to let a Sympathetic Murderer get away with it.
Fridge Brilliance: Poirot's friend thinks they are dealing with "a crime of passion," which leads him to suspect that the murderer is either the Italian or one of the women on the carriage. The murder was actually premeditated and planned down to the last detail - and the force behind the idea, the recruiting and the planning was a woman.
Poirot's decision at the end may not only have been moral. Let's see him attempt to arrest everyone on the train... Although, he could just have waited till the train came into a station and carried out arrests there...
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.