There were three big dings on the signature drink when I used to judge.
The first was not being able to taste the coffee. This can be a heart breaker -- do not use a Sidamo with apricot flavors with apricot juice, or a Brazil with cocoa flavors with a chocolate drink, since the judges won't know whether they are tasting the coffee or the additives (and when the judges start arguing, the competitor is usually screwed).
The second is poorly thought out complications: umbrellas, garnishes, utensils and El Bulli style trick plates will usually be a disaster -- if you can't tell the judges how to drink it in one sentence, change your plan; if the judges might look ridiculous or do a pratfall while drinking it, change your plan.
The third is getting thrown by the degree of difficulty. You can get complicated in preparing the drink, but you will get completely slammed in all the presentation scores if you ever seem hurried, flustered or get in a sweat -- make absolutely sure that your routine includes only things you can pull off with masterful ease. More difficult drinks will score higher, but only if they are pulled off with
sprezzatura.