I have read partisan levels of division on this notorious short. I will add my view to the debate.
The 'success' of the piece, if that's what garners acclaim or opprobrium depending on your perspective, is for me in the images that linger long after the film itself finishes. Most films finish and you think, just.....nothing. Nothing stays with you - nothing comes back to you, it's just THERE. Here however, for me, odd moments return to the mind's eye and can have an unsettling effect, as if the director (shackled now to being a 'cultural' stereotype of the highest order who will polarise opinion faster than you can say Melvyn Bragg) knew that these few images were what would remain, and this only - the beach and the claustrophobic room. It's appealing to childhood fears - there's someone there at the door who will help you. There's someone there...you're OK, or are you? You're on the beach. You notice it, you feign ignorance, you know, you fear, you think it's after someone else. No, it's after you. It could be Hemingway, couldn't it?
I think this is the real essence of horror. No horror film will be bedecked with 90 minutes or so of abject terror, that's not the idea. The zeitgeist is in what stays after, weeks, months, sometimes years after, moments when you have to relieve yourself in the night, walk downstairs in the dark, conjuring up some half baked idea that there's someone/thing 'in the room' - at moments like this, which, let's face it we all have and are now trying to deny, images of Hordern on the beach, with that THING behind him, will leave you wondering if it will come closer, and, more worryingly, if it will come for YOU, my friends....