Dane facedness refers to the plane of the skull in relation to the plane at which the muzzle sits. Danefacedness is also called "down faced".
Refer to the following photos of one of my girls. The first photo is correct for the boxer head with regards to the planes of the skull vs the plane at which the muzzle sits.
I took this same photo and modified it a bit to make my bitch appear to be "dane faced". You can see how the angles have changed.
This fault greatly affects expression and alters what the breed was originally bred to do. A dane faced boxer would not be able to breathe as easily while he was holding his prey, because his nose would be shoved right up against the prey's skin.
Similarily, a poor ear crop can affect the expression, although this is not a genetic, or a breeding issue, rather it is an issue of the skill of the vet (or lack thereof) who performed the surgery. A proper crop will enhance the boxer outline and make the overall appearance more elegant. The bitch above has a proper, tapering "show "crop.
The stop is the part of the boxer head that occurs right between the eyes where the muzzle meets the skull. There should be an indent there. The indent should not be so deep as to loose your thumb in it (if resting your thumb on top of the dog's muzzle), but it should be just deep enough so that the thumb can rest comfortably in it. Lacking in stop means that the head is totally devoid of this indent, which is very incorrect for our breed.
The tail should be set at 1 O'Clock when erect. A low tailset would appear closer to 2, or in extreme cases, even 3 O'Clock. You would likely also see a rounding of the croup (the area right before the tail, from the hip to the base of the tail) that would accompany this fault.
The topline, or backline, should be straight and slightly sloping. The topline extends from just behind the shoulders to the base of the tail. This line should not be curved or arched (swaybacked).
Front and rear assembly are a bit harder to understand (or at least, I'm still trying to understand proper front assembly, as certain issues can be very deceiving). This refers to the actual angles of the bones that make up the shoulders and rear of the dog. If you can imagine the dog's skeletal structure in profile, and draw mental lines where all the bones would fall, you will note that there are angles at which those bones meet.
With regards to proper front angulation, the angle between the scapula (shoulder blade) and humerus (upper arm bone) met should be at a 90 degree angle. A "straight" shoulder would be one at which the angle was less than 90 degrees. The rear is similar in that proper angulation should occur between where the hip meets the pelvis and the femur meets the fibula. A "straight rear" is a rear that is devoid of these proper angles.
Straight fronts and rears make it hard for a dog to move properly. The boxer is an agile breed, and should be able to move with a certain smoothness and efficiency that is virtually impossible to achieve if the structure is not proper and correct.
Many feel that dog shows are simply "doggie beauty pagents". This is simply not so. As you can see by the indepth analysis that goes on behind the scenes, these are the things that run through a boxer judge's mind while they are evaluating the stock that is presented before them in the ring. They tell this in a variety of ways - by evaluating the dog as he stands (structure) and evaluating the efficiency or lack there of in how the dog moves (movement). If any one of the things that I mentioned above are off or incorrect, the chances that a boxer will be able to perform the job that he or she is able to do are greatly reduced. Dog shows are about preserving the breed and maintaining the integrity of the purposes for what the breed was originally created to do.
Hope this helps.