The length of the back is a separate issue - and most people don't have a good eye for length of back.
There are two parts of the back that make up its total length - the saddle piece, and the area behind the saddle. The saddle piece is the area behind the withers where the saddle is/will be. The area behind the saddle is far less supported, and length there, can be more of a problem than length in the saddle piece.
The 'length' of the back (overall) is relative to the other proportions of the horse - leg length, neck length. All 3 need to work well together. While there are rules for various systems of evaluating horses, there is room in sport for some variation in these proportions.
For example, a slightly longer backed horse may gallop rather well and do very good lateral work - as long as he doesn't have other faults that might add up and cause an issue, such as weak, narrow or stiff hind quarters. A shorter backed horse might be easier to bring together and balance, thought it might be harder for it to stretch out and cover ground - again, if it has other conformation faults. Each type has advantages and disadvantages.
It's important to remember that conformation faults tend to occur in groups, creating an an overall weakness. For example, a weakly muscled, long area behind the saddle, plus hind quarters that push out behind the horse, and straight hocks. It's important to learn to see the overall picture.
Too, each horse always represents an overall type - heavily boned, heavily built, vs very tall, leggy and fine. It is not so much that there is any specific 'conformation fault', it's whether the type is suitable to the intended use. For example, a draft horse can have a very good type - for a draft horse. But you probably wouldn't enter it into the Kentucky Derby. Type doesn't have to be that extreme to affect the use. People need to learn to see the overall type, and instead of criticizing every horse because it doesn't fit, say, for Western Pleasure or Reining, see what it IS good for.
To get a better eye for these proportions, you can learn to squint at the horse to blur out all details, and you will see more the overall proportions, instead of the details.
Keep in mind that it's very hard to evaluate a photograph. Photographs tend to distort proportions. A photo taken from below makes the legs look longer, a photo taken from above makes the legs look shorter. If the horse is stood up at an angle to the camera, his back can look overly long or overly short. In Quarter horse pictures, there's a tendency to pose the horse with its hind quarters at an angle toward the camera, to emphasize the hind quarters and make them appear larger. This angle makes the back look short when it is not.
The biggest error I see is people (esp on the bb's) declaring a horse has a 'short back' when it does not. This is probably the commonest error when people try to critique conformation. Often the photo angle, a deep chest or long neck wil fool the observer into thinking the back is 'short' when it isn't, but there is no real pattern to what causes people to not see the length of back.
Comparing the length of the horse's body overall, to leg length, can help. With a lot of practice and being corrected by a more experienced person, eventually a person will get an eye for this proportion.
Last edited by slc; 02-08-2011 at 08:05 AM.