Many people like a slightly less intelligent horse for a first driving horse. The really smart, active, sensitive horses take more anticipation and forward thought on your part. If you are new to driving, you've got enough on your mind about how your hands are moving, where the cart is, where it will be after this turn, etc to add on what the horse could possibly be thinking you're going to ask him to do at the next cone.
I spent years driving drafts that were very good at their jobs but took a fairly heavy hand and the extent of their spook was to prance while still going the speed and location I had asked. I just started my Arabian to cart it it just amazes me how sensitive he is to the slightest rein cue. I chose him as a cart prospect because his is incredibly smart, sensitive, likes to move, is quite bold, and absolutely demands to be the center of attention.
Too many people blame the driving horse for a hard mouth, but in reality, it is the trainer/driver who is at fault. Heavy hands is predominant in the draft world and I can tell, who has to drop those lines to control those horses. My mentor, who has driven for more years than I have been alive said, and I agree, you can tell how good and experienced a driver is by the placing of his lines. I can drive my team with very little hand movement, it is 90% voice and that is why we use to go around our area helping people and teaching that a draft horse is not some dumb animal, they are large yes, but they are far from dumb nor do they have to be roughly handles or treated differently.
This is my own .02 worth from what I have seen helping out at several Percheron Congress shows here and in Iowa, as well as showing my own.