I think if the background were a little more discernable, the hilights not blown out it would help tremendously. Also (or maybe just alternatively), maybe a different perspective, where the spines draw your eyes across the photo.
Great idea and subject, though I think that this shot could be improved by getting up close and personal with the cactus. The background is a little distracting and it adds nothing to the shot. A more unique perspective could bring a lot of interest to this shot too. I'd love to see a macro shot focused just on a few rows of spines to *really* emphasize the patterns here.