Great feedback guys! I really like what the gradient adjustment (contrast up, brightness and saturation down) did to the bottom of the image. The blurred legs and hoofs stand out now to a degree that much better matches their importance for the shot, and I very much like how brown dirt ended up very close in tone to the horse and rider's skin.
Kevin, I also like your idea about adjusting the background on the right of the rider to better match the left. The result seems to be greater isolation between the rider and background. Nice! I can't really tell if the camera was tilted or my pan motion was, but it does indeed look better to me rotated slightly counterclockwise. To me, the rider ends up seeming more properly forward in his saddle.
Ken, I'm always a bit worried about too much vignette, because whenever I notice vignette I find it distracting. I try to find an amount that works to emphasize the subject without actually being easily evident. Here, as is probably often the case, I was being over cautious. Yes, haha, I had already applied a slight vignette. 8-) I think all it did was slightly reduce the reverse-vignette-like pattern of brightness that was already present in the background naturally. So I went at it again and ended up lighting the middle of the image as well as darkening the edges. I'm curious if you find what I did too subtle still. That would not surprise me!
I really appreciate the help! While I was down in South America I saw that a photo contest web site I like opened a contest with the theme "South American Culture", and I plan on entering this one once I'm done tweaking. Here's version #2, and I'm curious how well you think this addresses your excellent feedback, if you care to have another go at it.