The shadow cast by the light is not pleasing. Colour is good, plenty of texture. Maybe if you had concentrated on a close up of the 2 on the left if would have worked better?
Good idea, not too sure about your F stop though. If it had been higher you would have had more of the picture in focus. Maybe the cropping too is a little harsh, would have liked to have seen some of the edges of the cacti.
I find the close-cropping of this unuseful - and lighting from the same direction as the camera has reduced the effect of texture in the shot - try sideways lighting, you'll get mote light and shade, and more interst for the eye. Good shots of cactus are rare - I think generally because they're such lumpy ugly things.
The idea of using a cactus is a good one. You have three main elelments but two of them are out of focus. It would be more appealing if all the main stems of the cactus were in focus. Cutting off the top of the middle cactus stem is distracting to the image..
Great idea, but the soft focus on the bottom part really hurts, I think. I would try to get closer to one section and really show the prickly parts up close. Also, I think a background with a richer or deeper color would be more effective.