Facial expressions really set some competitors apart from the crowd. Dancing is about storytelling, so it makes sense that not staring blankly- or, even worse, scrunching up your face while trying to remember choreography, would be a huge tool towards improving your dance.
Remember to make everything much larger than life- you have to really put effort into your expression. If it is a performance, wearing makeup is also important. Make sure your face matches both the music and the story you are trying to tell. For more resources, see the links below: