To start to understand dressage fully and what the problems are with the sport, we have to go back to the very basics.
What we see in the arena and what is really happening are sometimes 2 very different things.

00ac_chacomoHere we see Chacomo in extended trot.

Salinero3
and here we see Salinero performing the same movement.

So, which is the better horse? Today most would reply without hesitation, Salinero. It is not hard to see why, he looks like he's really covering the ground, whereas Chacomo just looks very extravagant, but not really gaining ground.
The answer lies in the head carriage of the 2 horses. Look again at the picture of Chacomo and note that the nose is what is known as slightly in front of the verticle - where it should be. When you look at Salinero, the nose doesn't appear to be in front of the verticle, rather it is pointing towards the chest. Remember - a horse can not place his foot in front of it's nose, so the length of stride is restricted by where the nose is.
So, the better horse is Chacomo, yes, it looks extravagant, but he is covering more ground. Salinero, by comparison, only looks to be covering more ground.
Also, still loking at the head carriage, we see a big difference. Chacomo holds his head higher than Salinero. This is not build or genetics, it is training. Chacomo is elevated in the front end, allowing his hind legs to power through the stride. by comparison, Salinero has more weight over his front legs, and is not bringing his hind legs through.

The reason for these differences is one word: Duchlaessigkeit.

Translated simply, this means "through" or "letting the aids through". A horse is Durchlaessig when:

It is listening to the rider, waiting for the aids and willing and able to obey them.

It accepts the contact without resistance and goes forward and sideways on the aids without hesitation.

The rider han condense the outline from behind into the had, so that both hind legs push straight forward and underneath the horse's centre of gravity.

There is no resistance and no delay, the horse responds immediately and submits willingly, getting lighter in front, and going over the back (rounding and arching the swinging back).
I will cover more on this term in my next post.