For a driving horse I want an agreeable attitude and a lively personality--I want some energy, not a slug that has to be pushed.
Conformation--long, well angled shoulder with neck well set on, long hip with a relatively high tail set, good length of leg and good clean joints--no meaty hocks (which is something many people don't seem to notice in Miniatures)--good length of neck with a clean throatlatch. I also do not want a wide built horse. People talk about wanting wide chests, but in reality your best moving horses do not have wide chests. When viewed from the front the horse should have a nice inverted V where his legs and chest join. You don't want both front legs coming out of the same hole, but if instead of a V the front legs form an inverted U with a very flat base--that is too wide.
People talk about a narrow chest meaning poor lung capacity, but in actual fact it generally isn't an issue. When I was 16 I got a Morgan gelding that had horrible conformation--his front legs did just about come out the same hole--we bought him because we felt sorry for him! In spite of that narrow chest that horse could out trot just about any horse he was ever ridden with, and he could go all day without running out of air--the very narrow chest did not hinder his breathing ability at all. He absolutely sucked as a show horse, obviously, but his conformation did not hinder his trotting ability.