I feel that the image could have been improved a lot by changing the viewpoint. The rocks in the background detract from the impact of the main subject. The mono effect makes in look a bit flat. What I'm trying to say is the main subject doesn't stand out from the background enough.
I think this photo is awesome. The backlighting here is subtle, I like that. The storm brewing in the background adds a lot to look at. I like your choice of b/w also. Nice work! :-)
This looks like New Mexico to me. Very nice in black and white, would like to see the color version as well. The mountains in the background really make this cactus seem even bigger. I love the hint of sun rays near the top.