hehehe, I thought I'd start one like this today and here it is already! I've got to go through some of my conformation books tonight and find some diagrams to copy for you guys, there's some really great stuff.
Ideally, the scapula (shoulder blade) will form a 90 degree angle with the humerus (bone between the scapula and the forearm). The line of the shoulder formed by the scapula itself should be a 45 degree angle. And the higher the point of the shoulder, the easier it is for the horse to bring that humerus forward and give you some true reach through that shoulder. Picture it this way- if the horse has a low point of the shoulder where scapula and humerus meet, the horse has all that humerus bone tucked away almost horizontal. He or she would have to sort of "drop" or "unfold" that bone in order to use it, and there's simply not enough distance between their body and the ground during a stride to allow them to do that. But a horse with a high point of the shoulder already has that humerus at an angle and all they have to do is swing it forward. Voila, reach!
In a riding horse, the angle of the shoulder is also important because you get shock absorption through the shoulder and front pasterns (and those two angles should be identical no matter what that angle is.) The steeper the angle, the more directly that concussion travels up the leg and jars the rider/pounds the horse's joints. Again, you want 45 degrees.
I will post later with pictures of some of our horses. Spyderman and Kody both have excellent shoulders, but my mother's Arab Bo has a very steep angle and the difference is obvious when riding him.
The new carriage driving book I'm reading also has some great information about how the conformation of the shoulder affects a driving horse's ability to move while in draft as well. I'll try to post it all! Complete with little lines and arrows and pointy highlighting thingies.
Leia