Strange how the desat makes the tack look like it's glowing...unless you did that on purpose. Who knows. I like that most of the image is blurred aside from the tack, but it seems like post-processing and seems over the top.
Ouch! But there is no tack in this image. ;) The redness of the tack speaks of the pain and anguish to come. The motion blur of the foot lets me know it is unsuspecting and the moment is eminient. The question is, does he do it? Does he step on the tack? Does he scream? I can't stand the anticipation so I have to go on to the next image. 8 for the tension created.
If not for the shadow it would have taken me longer to decipher what this was. If the foot were shown at a slightly different angle or with better focus this might require less thought and have a greater impact.