Never in the history of film has there been a more satisfying death on screen: There's a bit in the documentary Not Quite Hollywood where Quentin Tarantino expresses exactly how I feel about the couple in this film: this movie's two goals are to introduce you to this couple, and to make you cheer their inevitable death. And boy do you cheer!
The horror elements actually come from all of nature: Many horror movies that utilize nature in their horror elements do so in the form of a single scene (like Evil Dead), or focus on one element of nature (like Frogs, or the Birds). This movie actually makes all of nature the stalker-killer, eliminating overly complicated exposition or redundant kills. The viewer is never sure what's going to happen, and that gives the film a layer of creepiness that can only come from your imagination.
One flaw [slight SPOILER]: The topic of abortion is somewhat ambiguously introduced, implying that nature has made it's mind up about these two because of their choice. But over all these characters are horrible enough to justify their ends, without adding any elements of "but she also had an abortion". (Honestly the couple terminating a pregnancy is about the only redeeming thing they do because they absolutely should not have brought children into the world together.) There is room to interpret this element as just another layer of how absolutely divided they are on every front, but other devices could have been used to illustrate that without leaving room for viewers to pin their demise on her having an abortion.
We really like this one: Maybe it's because both of us have worked customer service type jobs most of our lives, but something about this one scratches an itch we didn't know we had.