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.
Esoteric Happy Ending: Or a Bittersweet Ending, or a Downer Ending, depending on your views. Grace killed her children, and they are all presumably forced to dwell in the house forever, separated from their husband/father and with no indication that there's a higher power looking after them. However, they have resolved most of their internal strife, the children are no longer harmed by the light, and Mrs Mills' general demeanour suggests that being dead isn't really all that bad. Grace even manages a small, grateful smile when she offers to get her a cup of tea.
Harsher in Hindsight: The twice-repeated "Stop breathing!" takes on a tragic new meaning after the final twist.
Rewatch Bonus: If you watch the film again after learning that Grace and her children were Dead All Along, you'll notice how alot of the things they do are common with what people usually associate ghosts with (erratically moving things, wearing creepy clothing, making noises/talking in acoustic hallways/rooms, not leaving their home etc.)
Squick: The "book of the dead" Grace finds, filled with photographs of corpses posed for portraits. What's amazing is that the film downplays this very real practice — In the film, all of the corpses are posed either alone or with others of their kind, with their eyes closed as if to be asleep. In real life, most such photographs involved living family members — and their eyes were very much open.