Which conformation faults are most serious?

I'm interested in hearing other peoples' opinions as to what the most serious conformational faults are in a horse. To keep things relatively simply, let's say that we are talking about a horse that would be used for trail riding, hacking, lessons and maybe a fun show or two - not a serious... show more I'm interested in hearing other peoples' opinions as to what the most serious conformational faults are in a horse.

To keep things relatively simply, let's say that we are talking about a horse that would be used for trail riding, hacking, lessons and maybe a fun show or two - not a serious competitor. We're concerned with longevity and with the horse being sound and useable as long as possible, not with which type of horse would do the best in reining, dressage or eventing.

For a horse of the above description, what do you think the worst conformational faults would be?
Update: Interesting about the jowl width, Mule, can you or someone else expand on the problems with that one? I'm not too familair with it.
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