Caution! Noob crit approaching!
Okay, enough tacky. I'm going to try to crit this as an outside perspective having no influence of the genre of 40k at all. Having not played or cared any at all about the games whatsoever. I'm going to try to crit the piece as it is. Not by what lore says it should be

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Okay, first off. Take a second look at what ndchristie's saying. Basically your piece seems a bit static, non active. I'm not saying you should scrap the whole thing, but at least change the perspective maybe a little bit. Then you could easily move the arm on the right side facing out to grab that orc enemy. It shouldn't be too difficult (riiight?). Also, this bothered me in the first piece you showed. The bottom leg, and the background cause my eye to look at everything else but the mech, which is what I'm supposed to be looking at. If I had this level of expertise, I'd probably change the perspective of the background to match the one in the left foot, or the left armor shoulder pad. Because right now I'm being dragged all over the place forcing myself trying to concentrate on the mech. This could also be fixed if you would increase the size and straighten out that right arm a little. Not only would it give the mech itself more attention, but it would put my view back in the center on the mech itself. I also think that it's a little bit dull. You helped it alot by adding the lighting from the flame thrower in some places, but that only helps a little bit. Another thing that bothers me is the smoke clouds. They look more like background art rather then coming from the heat stack flames. (And I'd be a bit more mangled if something that size grabbed me!)
Anyway, that's all I've observed. I think this is going to look very cool, regardless of what your going to do.
Keep it up!
Some quick refs I found online. One's a deviant piece.
http://www.danscottart.com/Images/Dreadnought2.jpghttp://warhammer.hardwired.hu/gallery/wh40k/40k_011.jpghttp://www.rushputin.com/gallery/d/5799-1/Dark+Angels+Venerable+Dreadnought+2.jpg