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.
Idiot Plot: If the characters would just take a look at Henry's actions, not be fooled by his obvious sociopathy and apathy, or just were able to figure out when he's acting, the plot would have fallen apart.
Inferred Holocaust: There is a strong possibility that Mark would end up being arrested and sent to a juvenile detention center after the events of the movie. Especially when looking at the fact that everyone, except Susan, has written him off as a nutcase due to Henry's manipulations. Also Susan's marriage would be in trouble the minute she tries and explain to her husband what happened to Henry and even if the former two doesn't happen she'd end up gradually resenting Mark down the road because she chose Mark's life over Henry's. She'd also be forever haunted over what happend at the cliff.
Narm: Henry falling to his death could have been pretty dramatic if it wasn't for the fact that there's no way in the world young Macaulay Culkin screaming doesn't remind you of Kevin McCallister.
That and if the adults act with some margin of intelligence.
Special Effects Failure: When Henry shoves his mother off the cliff at the end and she catches a branch halfway down, the cable holding her up is clearly visible.
What Do You Mean, It's Not for Kids?: Many critics decried the casting of Macaulay Culkin in the role thanks to his kid-friendly Home Alone series, many children hero-worshiped him and marketing research indicated extreme interest by children in the film entirely because of Culkin's presence. If the number of anecdotes by people mentioning this film gave them nightmares indicates, they probably saw it anyway in spite of the R-rating.
The Woobie: Definitely Mark, he lost his mother. Then it looks like he's going to bond with his cousin Henry, only to find out that Henry is Ax-Crazy. Then when he tries to warn everyone about Henry, they think he's the one who's crazy.