Like danastark said, depending on the breed or cross you get of draft, the trot can be pretty rough. IMO Percherons and Belgians tend to moved a lot heavier than Clydesdales and Shires. But that is very subject to the individual horse, and some of the nicest moving horses I have ever seen have been perch/TB crosses. However, even the roughtest trotter, in my experience, can be taught to collect back into a nice, lightfooted jog. My gelding is only 1/4 Clydesdale, but he is built very heavy - if I hadn't seen his parents I would think he's part Perch. He hit the ground like a jackhammer when I got him, but it took me less than a week to teach him to move much more lightly, and now I can sit his trot nicely. I have never rode a draft or draft cross that didn't have a great, rocking horse canter. I think because the gait hasn't been messed with much in the breeding, as it's not generally used for working purposes, it's pretty constant.
As far as comfort, in terms of their size, I presonally find them more comfortable than narrow, high withered horses, especially bareback. However this same feature can also make them much harder to fit a saddle to. Buying online is opened up a lot of doors for getting a hold of draft tack, but make sure you are aware of return policies if what you get doesn't fit - and make sure you have a professional help check fit on your saddles etc. I don't notice discomfort due to the greater girth, but I think some might.
Last edited by Indyhorse; 01-30-2010 at 06:29 PM.