Here's the AMHA standard.(I got tired of digging to find AMHRs)
AMHA Standard
General Impression: A small, sound, well-balanced horse, possessing the correct conformation characteristics required of most breeds, Refinement and femininity in the mare. Boldness and masculinity in the stallion - the general impression should be one of symmetry, strength, agility and alertness. Since the breed objective is the smallest possible perfect horse, preference in judging shall be given the smaller horse, other characteristics being approximately equal.
Size: Must measure not more than 34 inches at the withers, at the last hairs of the mane.
Head: In proportion to length of neck and body. Broad forehead with large prominent eyes set wide apart. Comparatively short distance between eyes and muzzle. Profile straight or slightly concave below the eyes. Large nostrils. Clean, refined. Even bite.
Ears: Medium in size. Pointed. Carried alertly with tips curving slightly inward.
Throat-Latch: Clean and well defined allowing ample flexion at the poll.
Neck: Flexible, lengthy, in proportion to body and type and blending smoothly into the withers.
Shoulder: Long, sloping and well angulated, allowing a free-swinging stride and alert head/neck carriage. Well-muscled forearm.
Body: Well muscled with ample bone and substance. Balanced and well proportioned. Short back and loins in relation to length of underline. Smooth and generally level top-line. Deep girth and flank. Trim barrel.
Hindquarters: Long, well-muscled hip, thigh and gaskin. Highest point of croup to be same height as withers, Tail set neither excessively high or low, but smoothly rounding off rump.
Legs: Set straight and parallel when viewed from front or back. Straight, true and squarely set, when viewed from the side with hooves pointing directly ahead. Pasterns sloping about 45 degrees and blending smoothly, with no change of angle from the hooves to the ground. Hooves to be round and compact. Trimmed as short as practicable for an unshod horse. Smooth, fluid gait in motion.
Color: Any color or marking pattern, and any eye color, is equally acceptable. The hair should be lustrous and silky.
It's very generalized IMHO, but as long as many breeders (and those that do volunteer to run the organizations choose to view it as a height registry not a 'breed' I don't see much changing.
Personally I've always wanted Miniature Horses recognized as a breed, but I do like the variety of draft, saddle, arab, quarterhorse - just miniaturized versions of the big guys. How cool would it be to go to a show of all miniaturize draft horses?