Here's the link, just so everyone doesn't have to search through the site:
http://us.games-workshop.com/games/40k/spacemarines/catalog/spacemarines/default.htm

Your sprites don't look bad, their forms look correct and they have nice poses. 

To me, however, they look a bit too much like a coloring book.  In order for the sprites to look more realistic, you'll need to incorporate more shadows/highlights and less outlines.  This will give them more depth and realism.

Also, I would define a lightsource more clearly.  This will also add to the realism.

So, overall, keep the outlines, but rework the coloring.  It's looking nice, good luck!