First off, great capture. I bet this was an awesome thing to watch!
Compositionally, the bike and rider are located at an effective point in the frame. It is balanced, and there is "room" for the rider to go, so it doesn't feel cramped or off-kilter. However, that bright light in the lower right competes with the rider for the viewer's attention, and that could be a minor distraction for some.
Technically, I'm sure this was a difficult shot to pull off in the best of situations, but it almost seems that the rider is too bright. while this effectively isolates him from the background, it almost gives it a "cartoonish" feeling.
I think, technically, you met the challenge. Like someone else mentioned, this would have probably fit better in the bicycle, or even fitness, challenges, but the dates don't always allow for that.
This is a great action shot.
If I need to further explain or clarify anything, please feel free to pm me.
Freakin´ AWESOME!!!! Doesn´t fit the challenge all to well.. but at this point, that really doesn´t matter - I know how hard it was to get this shot in the first place.
Keep the mtb pics rollin´!!! I´m looking forward to seeing who took this shot!