A long, sloping shoulder is important for any type of big front-end motion in a horse of ANY size, from miniature to draft. You can find good examples of shoulders in Hackney and Modern Shetlands, since they're bred specifically for big front-end action. Miniatures are not always bred for dressage or saddleseat action specifically, so their shoulder conformation is all over the board.
There are a lot of other skeletal mechanics that affect front-end motion, not just the shoulder. The length of the forearm and pastern affect movement, as well as the hoof itself (which is why you see such crazy shoeing sometimes). It will make a dramatic difference between a dressage horse with outstanding extension or a saddleseat horse that breaks above level. Neither one is "better"... just depends on what the horse's discipline is. In the minis, it will make a difference between your Park horses or your Roadster horses.
Here is my Modern Shetland gelding... his shoulder is very, very nice (although certainly not the most extreme conformation out there compared to some Hackneys or Moderns) so take a peek:
And then some examples of minis... this little 32" stallion I owned has a good shoulder for a mini and I will then show what kind of action he had, since it was not extreme like the above horse but it was Country Pleaure:
And one last one, that's kind of fun since we're talking about conformation... conformation IS skeletal... it doesn't change too much over a horse's lifetime although conditioning of the muscle and fat can improve the overall appearance and attractiveness of a horse. So if a horse is born with a good shoulder angle, he'll generally keep it.
Here is my Shetland gelding in almost the same location and pose, at age 3 months and again at 3 years:
Hope any of this is helpful! You just have to keep staring at lots of different horses and breeds to "get it"!
Andrea