I have had coaching lessons with many adjudicators and ask every one of them what they look for.
Number one is topline. They can tell the most from it. If your footwork, frame or timing is off they can tell just by looking at your topline. If everyone is equal there then they will look at the feet. If equal there then they will look into styling, etc.
Judges mark the dancers who "offend them least" according to the late Scott Montague.
I do not believe that people are marked or not based on their body. However their dancing CAN be overlooked because they are not wearing costumes that are appropriate for their body. Both men and ladies should keep their problem areas covered. Ladies should always be in panty hose unless they are extremely tan and toned. Even then, I'd still recommend it. Everyone's legs look better in hose, plus it keeps the spanks from riding up which is a MAJOR distraction from your dancing.
Other big no-nos: hair in your face, nude colored spanks (everyone spends the whole time trying to figure out if you're wearing any, and miss out on your awesome dancing), glitter (just looks like sweat under the lights).
As far as seeing many of the same people called back, there are a couple factors. One is because they probably deserve to be there. Judges do call also call people back that they know, that is true, but its not a politics thing generally. It's because they know their dancing. If they have someone that they already know if a fabulous dancer in a semi-final they are going to call them back without really watching them. This allows them to spend more time watching the dancers they don't know to see who else deserves a call back. When it gets to the finals then they'll spend more time with each couple.