This really is a post with so many different views. There are some things that I consider abuse in one person's hands but not in anothers. I would be comfortable using a twisted wire snaffle for a short time because I know how to use it properly (though other people may not think so), though I choose not to. However, many people have no business with anything harsher than a french link snaffle in their horse's mouth. I agree completely with Ricci,
When it becomes more than discipline and stems from anger
. I personally believe that some horses do occasionally need a firm smack to regain their respect. I have smacked a horse on the nose or pinched their lip for nipping me and it never resulted in them being headshy. I consider these things acceptable because it is the same way in a horse herd. An offending horse will be warned with actions and sounds, then physically reprimanded with either teeth or hooves. I will warn with a firm "EH" or "NO" and make my posture more assertive before taking it to a smack. There are certain things that a horse should never be punished for; being afraid, or showing pain or uncertainty, or being "stubborn" about picking up a new thing that I am teaching them. After they know what is expected, it might be a different thing. It is a case by case basis for me. Dobe might do something and I smack him on the shoulder whereas Denny would do the same thing and I just back him out of my space for a few feet. It all depends on the horse, the circumstances, and the environment of the situation.
However, there are certain things that I consider abuse no matter the situation or circumstances.
-leaving bloody sides from spurs
-leaving bloody mouths from bits
-hitting a horse out of anger for any reason
-hitting a horse for no reason; ie, when they haven't done anything wrong
-leaving a horse with no food or water
-leaving a serious injury untreated when it is known about
-tying a horse with his head in the rafters (seen it happen)
And there are some other things that I can't think of right now but my ideas of what is and is not acceptable treatment of a horse are pretty black and white.