Well, for someone who calls themselves 'average', I think they are rather excellent!
If I was to really nitpick,
IMHO, I would pick out just a couple of points:
A) The white looks just a little too thick on some of the helmets, most notably the beakie, but it's not too much to write home about - just a touch on the thick side.
B)The blue, whilst it's all down to your own personal taste, looks a little more crimson fist blue to me - possibly a highlight with a more vivid blue would bring a sense of depth to the overall effect.
Aside from those two points, I couldn't fault the rest - they look crisp, clean and dynamic
Regarding the marble, obviously your going for a white marble effect, but at the moment it looks a little too much like white with poor coverage, and not enough like marble with veins running through it. There is a good marble tutorial here on chest of colours:
http://chestofcolors.com/how-to-paint-marble-bases/
I've never tried it myself as it didn't suit my Salamanders Terminators and it looks quite complex - I think you may find it hard to apply to a small area such as a storm shield. However, a substitute may to add a web of thin lines of orange/brown/tan lines to your shields with a wash of Agrax Earthshade pooled in areas to simulate larger veins. I'm not sure how well that would turn out, but it may be enough to get your creativity flowing
Looking great - keep it up