1. I don't find the design on the wall distracting, I think it adds a good visual texture and cements it in my mind as the bounds of the board. It gets a little busy looking on the zoomed out view, but I'm not sure how close the camera will be in the game.
2. I didn't get that the objects are destructible, they look pretty solid. I think they definitely need to be cracked into chunks to look like they're ready to be blown apart.
3. I don't know if you need to increase the contrast of objects, but maybe the contrast of objects with each other? Right now you're working in a very tight monochromatic color scheme. Are there gameplay differences between the different kinds of pillars/obelisks and whatnot? You've got everything in a very tight sliver of the color wheel, you could probably broaden the hues even just a little to help distinguish different objects.
4. The design of the enemy tank doesn't really convey "tank." It looks kind of like a food cart or something, it doesn't have a clear and threatening canon. I feel like the design should convey to the player, "don't stand in front of this end." And again, I don't know what the player's camera will be doing, but a lot of the visual interest is on the sides, while the top is pretty much a rectangle, so it's not that interesting from most the the screenshots.
I think your work so far looks awesome, so keep it up! If you're interested, I found James Gurney's writing about color wheel masking to be really helpful.
http://gurneyjourney.blogspot.com/2008/01/color-wheel-masking-part-1.html