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Those comments were maybe said in a rude way, but they're right. The left leg looks like it has no bones, it's curve is very unnatural. By shield they mean that that abs/tummy part looks flat, like the character had a shiled under his suit. In a suit as tight as that, the man parts' silhouette should be visible; he has a crotch of a woman now.
Hey man good beginning!
I would say get the book "Figure Drawing for All It's Worth" by Andrew Loomis to help you with Anatomy. Also study Bridgman and Robert Beverly for great instruction on anatomy as well. Also draw from life, paint from life, get "Perspective Made Easy" by Norling; also study the old masters, read art books, watch dvds on art, and always keep making pieces as you do all of these other things. That should help you. Okay now onto your piece-
1. Your piece is comic in nature (of course) so the pose overall reads well; just fix the right leg and the abdominal wall and you should achieve a better result.
2. I would love to see this in a non-comic style; that way you could really explore the value relationships relative to your light source you would have chosen. The comic style is great, but rendering with those harsh dark values isn't helping your form much; a lot of experience is need to render convincingly in this style of comic art. Also it will help you by not being constricted to that "Marvel" style of rendering for printing sake.
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