I have been around quite a few drafts and all of the ones I know have been very sweet and caring. I haven't had to teach my draft mare manners though, I just had to remind her that she weighs 1,000 lbs more than I do. Otherwise she takes care of everyone around her and is especially careful around kids.
I have only met two draft horses with personality issues, I work with a six hitch of Clydesdales, but other than that they are incredibly sweet. The first was a stallion that was constantly grumpy, and the other was a mare, it turned out that once she was sold for a pasture pet, she just needed more attention, but no one wanted to love on her because she was ill tempered. Now she is sweet and loves people. Others I know are so gentle, a stallion that was in a hitch with mares and was fine, drafts in their teens being retrained to ride, the draft horse is a truely remarkable type of horse, in my opinion they are generally the "Gentle Giants" of the horse world.