Halter is a beauty contest. Lots of muscle, shine and straightness. The horse does have to be presented perfectly, meaning set up square so the judge can look at it like a picture, so if the horse is moving around, they can't judge it, the handler is a part of the picture as well, not a big part, but it counts as well.
Are you sure it's halter or maybe it's showmanship? That is different, doesn't matter how the horse is put together (although it doesn't hurt to have an attractive horse), there is a pattern to follow, and the handler is judged on how well they show the horse to the judge. There is a quarter system when the judge is presenting the horse and the judge may ask the handler a horse related question. Technically you could take a range colt, groom him to the nines, teach him to square up, trot with a handler, pivot, back, etc and win.