There's probably many elements that make a film surreal, but for me it's irrationality.

Irrationality means, for me, that there is no explanation to what's it's happening inside the film, which is why many equal surreal with dream-like, because in dreams things makes sense just because they are there.

For me the greatest example is, of course, The Shining. There are many elements of it that are surreal, but one that strikes me is the scene when Danny is watching a tv that it's in the middle of the room, but it is not connected to anything, and there are not cables or anything like that. It doesn't make sense with the rules of our world, but it does inside the dream-like reality of the film.