I appreciate all the constructive criticism, but a lot were things I considered when doing the design and then made decisions about.
You're not going to get any better as an artist if you just disregard criticism like this :/
I mean some of it could be chalked up to your artistic sensibilities, but a lot of these ideas would really improve your piece. Really simple things like making the turtle look like it's walking, or moving the smoke so it's drifting behind the gun, would make an incredible difference.
Remember that people aren't attacking you or your art (well, some of them are), they're trying to help you out. They're the ones buying the shirt after all :P You should resubmit this piece after taking some of their critiques into account!