For example compare a Shetland Pony to a Thoroughbred. There is no comparison you say, but there is! If they are each in proportion they can both have their breed's 'ideal' conformation.

What I will try to illustrate here are some of the common conformational defects.

A correctly proportioned horse, regardless of it's breed, will be better balanced, less prone to unsoundness and more efficient in his work. However this does not mean that you should dismiss a horse with conformational defects immediately. What you should do is to identify his problems and decide if they will restrict him from doing what you actually want him to do.

Obvious cases where near perfect conformation would be preferred are, if you wish to compete successfully in any type of showing classes. This does not mean that you couldn't compete, just that to be repeatedly successful conformation is key. I have seen cases in working hunter classes where the winning horse has poorer conformation than the second placed horse, but the winner was the only one to go clear in the jumping section!

Starting at the front:-

HEAD
Look at the horse's head, is it in proportion to his overall size? If it is too large this adds extra weight to the forehand, which is already carrying 60% of the horse's total weight, and upsets the balance. Likewise too small a head tips the scales in the opposite direction.
EYES
With the eyes many people prefer them larger, as small eyes with a lot of white round the edges are said to indicate a horse with a mean streak.
NOSE
The nostrils should be large and wide, to aid the intake of air during work.
NECK
A long neck is associated with speed, where a short neck is associated with strength. Hence the Thoroughbred, generally used for racing, has a long neck and a Shetland Pony, renowned for being one of the strongest in the equine world in relation to it's size, has a short set neck.
'EWE-NECK' refers to when the top line of the neck is concave and the lower line convex.
SHOULDER
In a good riding horse the shoulder should be well sloped. Typically the angle from the point of the shoulder to the point of the wither should be between 40 to 45 degrees. This allows the horse to take longer, more fluent strides. A straight shoulder would produce a shorter more restricted action and could cause jarring to the fore limbs and a less comfortable ride. A horse with correctly proportioned neck and shoulders is described as having a 'good front'.
LEGS
The forelegs should be straight and strong, short cannons indicate strength and the amount of bone indicates weight-carrying ability. This measurement is taken around the cannon below the knee. 'TIED IN BELOW THE KNEE' refers to when this measurement is less than further down the cannon.
Continuing down the foreleg to the pasterns, which should also be slightly sloping, but not too long as this denotes weakness. Upright pasterns jar the whole leg and give an uncomfortable ride.
With the hind legs, if a line is dropped from the point of the buttocks to the ground, whilst the horse is standing still, the line should touch the hock and run down the rear of the cannon bone. 'COW HOCKS' are those which when viewed from the back turn inwards and 'SICKLE HOCKS' are where when viewed from the side have a concave line in front of the hock and a slanting cannon bone.
At the move the horse's legs should move in two parallel lines, without crossing or dishing (where the front feet are thrown outwards at trot).
HOOVES
The fore hooves should be well rounded and open and look like a matching pair, opposed to narrow and donkey shaped. The hinds should be more oval in shape. PIGEON TOES are where the toes turn inwards and SPLAY FOOTED is where the toes turn outwards.
BACK
The back should rise slightly to the croup and be well muscled. This is where the horse's strength lies so a back that is too long can be a cause for concern. Other problems are 'ROACH BACK' where the spine has an exaggerated upward curve and 'SWAY BACK' where the back has an exaggerated hollow.
Depth through the girth ensures plenty of room for the lungs to expand as these govern the athletic ability of the horse.
HIND QUARTERS
The croup should be the same height as the withers and not too sloping, although this does show the ability to gallop.
The hind quarters are the horse's 'engine' and should be strong and well muscled.