SK said that the first half is like Joe and him and the second half is pure Lovecraft. A lot of people compare it to Children of the Corn also.
My feeling is that unlike Lovecraft or Children of the Corn, it never specifies exactly what the evil is, or what it does or why. In a horrorstory as soon as you get some explanation or an evil gets a name, it makes it easier to pinpoint what it is about. Even when that explanation doesn't say very much by itself. In Children of the Corn you don't learn a lot about He Who Walks Behind the Rows, but at least you can give a name to the menace.
In the Tall Grass doesn't do that. It refuses to explain much or anything at all. I asked the director whether he had his own theory about what is going on and whether there is some deeper meaning behind it all, and he joked that he had, there is, but he won't ever tell.
Whether you feel that explaining nothing at all just puts on more mystery or you think it's weak plotting is up for debate. But in King's case there is always so much else going on besides the main plot (the pregnancy in this case for example), that the absence of a real clear plot is not necessarily too jarring. The result is that you tend to obsess over it more, wanting to find some explanation, that may not even be there.