1. Normally, I would say bad things about a movie like this. I would say that the story is unoriginal (a family moves into a house in the woods, and things in the woods aren't happy). I would say the movie steals pieces from multiple films (I lost count at six) and mixes genres. I would say it contains hokey metaphors about environmentalism and the power of a mother's love. That's what I would normally say.
2. But I can't say bad things about this movie. Instead, I have to say that this is one of the best horror films of 2015.
3. The movie is based on old dark Irish mythology. Writer/director Corin Hardy takes a risky move by mixing in elements of Evil Dead, The Fly, The Thing, Straw Dogs, and Aliens, among others. Hardy obviously loves these genres, and he has the skill to show these references in a plain and unapologetic way, but which doesn't insult the original films. This is something his contemporaries like James Wan and Leigh Wannell haven't figured out, which is why The Conjuring and Annabelle are just patchwork quilts of disappointment.
4. The visual mood and atmosphere is dark and beautiful, ethereal and off-putting. It fits the subject perfectly.
5. The camera seems to have a mind of its own, sometimes focusing momentarily on mundane things like someone putting down kitchen cutlery, or tapping fingers. This adds to the feeling that something isn't right, but you can't figure out what.
6. The movie is not entirely serious - since it's partly an homage to other horror films, Hardy can pull off a little tongue-in-cheek attitude. The flaming scythe should be added to the collection of iconic horror weapons.
7. The movie is kind of violent, but it's not super bloody. It's more... gooey.
8. Hardy tries to update the mythology with a sort-of-scientific explanation. It muddles things up a little bit, and wasn't really necessary, but it is an intriguing idea.
9. What Hardy has managed to do with this movie is take elements of other movies and forge them into a tool that taps into the deep primal fear that exists in every person and which inspires all those other films. It doesn't matter that the movie is kind of predictable - it gets you anyway.