It Follows is a well made, (mostly) well acted horror film that relies more on suspense and some interesting camera work than jump scares. But, it is built around a rather antiquated horror-film-theme--that sex is dangerous. It almost plays like a mid-80s slasher film just with more modern cinematic sensibilities. The danger in It Follows...
The danger in It Follows is rather literally tied to sex. Potential victims can pass the danger along through sex, which both promotes casual sex--surely, if you must pass on the danger to survive, you'd rather give it to a stranger than someone you know and love--and suggests repeatedly that sex at all is a bad idea.
We are never offered an origin story for It, but it seems important, and telling, that the first time we see It it is a naked woman and another time we see It it is a half naked woman, her clothing torn. There seems to be a suggestion of origination in sexual assault, which ties so nicely into the casual sex line that it seems a bit too on-the-nose. Furthermore, toward the end of the film, when the main characters attempt to fight back against It, there is a Freudian angle in It appearing as the father of the lead character--Maika Monroe, carrying the heavy load while supporting cast members are occasionally superfluous... I forgot to mention that, as part of the sex-is-dangerous theme, It can appear as a stranger or as someone you know. The danger is everywhere. It's like an abstinence-only PSA got together with a horror film... In fact, other than a bit of CGI trickery here and there, the script would probably sit quite well in the 1980s, or the swinging 70s.
The film plays on a strangely anachronistic nostalgia, as well, but not for the 80s or the 70s but the 1950s. Characters regularly watch old science fiction films, including at a silent movie theater equipped with an organ for accompaniment. Like many a slasher film, the political bent of It Follows is quite conservative. So, of course the film yearns for a simpler time, for childhood. Jay (Monroe) and Paul (Keir Gilchrist, who seems vacuous in some scenes and quite brilliant in others) talk repeatedly about the time when they were younger and they kissed. Ultimately, this is a lead up to their having sex near the end of the film, but it also reinforces the idea that the more innocent past is the better reality.
All of that being said, It Follows is definitely worth viewing, especially if you are a fan of horror films. Imagine it's a film from the early- to mid-80s and it will play so much better, though.