Horizon lines and subject overlap are a couple of topics that have been raised in this interesting thread. Here's a screenshot from Sam Abell's gallery page (samabell.com) that I'm sharing to offer a different perspective.
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Abell makes brilliant use of horizon lines in his photos and they often intersect with the subject. They key, IMO, is where they intersect. In the above photo - one of his most famous - the horizon intersects with the cowboy who's squatting to castrate the bull. But the line does not directly intersect a joint. A line cutting across a person's neck, waste, or another joint gives an impression of decapitation or amputation. In this photo, the horizon line intersects just below the shoulder and does not give an impression of severing a limb.
Several subjects overlap in the photo. Importantly, however, no faces overlap. The cowboy on the horse, the two cowboys in the middle ground, the person in the foreground at left...their faces are unobstructed. We even see the horse's face and that of the cow between the red bucket and the cowboy in the middle in the foreground.
The faces we don't see are those of the animal being branded in the midground, the bull being castrated, the cowboy performing the castration and the man carrying the bucket.
I would argue those omissions strengthen the photo. If we could see the face of the animal being branded, that would demand our attention and be a distraction. The same goes for the animal being castrated. The cowboy at the center of the frame is not, himself, the subject. It's the act of castration that is, which is why the scalpel held in that man's mouth but clearly visible against his shadowed face is so important. If the scalpel weren't so prominent and we saw the cowboy's face instead, the photo wouldn't be as impactful.
The red bucket is what makes this photo so powerful. It adds color and is, itself, an equal subject. It substitutes for the blood we do not wish to see and makes clear full impact of the activity that's underway.
The one "imperfection" in the photo is the horizon line cutting through the elbow of the cowboy in the white hat and, it could be argued, even that contributes to the image. It leaves an impression of the arm being removed at the elbow. It provokes a feeling of tension or conflict in the viewer. That feeling echoes our reaction to what we're seeing: an animal's testicles being removed.
It's amazing to me that Abell managed to so (nearly) perfectly compose this photo and wait until just the right moment to press the shutter release.