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I've explained and demonstrated a few things that one should take into consideration for concept art. It's a mixture of various things, as I've used the provided art of InvertedVintage (at ModDB) as an example. He asked me for feedback and this is my unterstanding of constructive critism. I've definitely forgot a few things, but this doesn't necessarily need to be my last guide.
I hope it helps some people out or at least entertained them.