Right. Gonna just try out this whole "Critique" feature here, as I'm strapped now for writing anything else.

Anyway, have I ever told you how much I love this?
No seriously. Every time I mention you to a friend, this is the piece I show them to say "look at this guy. Yeah, I know the icon offends you, but look at that damn picture!"
I remember seeing the WIP you posted a while back, and having to pick up my jaw thereafter. Even then I knew I was looking at something really truly amazing.
First off, the most immediate thing a person looks at here is the guy's face: his expression, with that half-glazed look in his eyes, speaks of a wisdom far-beyond any mortal comprehension - like he's seen the very stuff the universe is composed of, and has come out missing a part of himself.
I cringe slightly every time I look at the myriad bits of metal which pierce his skull and bump up under his skin; every point of insertion is red with irritation, as it likely would be. The shading is absolutely astounding, as is the lighting... in fact, looking now at the creasing of his skin, I just now gained a new perspective on his face.
There is, nevertheless, something missing from the face... it is not entirely realistic, and I suspect it might be due to the nose: that is the one area where your shading seems to fail, as the bridge appears to be going somewhere entirely different from the tip, and meanwhile the lighting there is slightly contradictory, as though there were two simultaneous sources acting on the sides of his face, without affecting one-another.
Traveling away from the head - if we dare - we are then promptly taken along to meet with just about every detail on this guy's armor. The metal and the piping around his neck is, strangely enough, stunningly-rendered and shines so magnificently.
Venturing from the neck, one finds they are met by that impressively worn and battered collar, then the stained skull icon built into his old chest plate. Every dent looks superb; every scrape and cut awe-inspiring. The mantra papers strung from his pauldron are a very characteristic touch, and look neither out of place nor poorly-wrought.
There is also the pauldron itself: every one of the demented little forms sprouting up from its surface looks as animated as the Marine himself, and the fires burning in their mouths look absolutely marvelous.
The negative space of this image is remarkable in that there doesn't seem to be any. You utilize just about every part of this image for something, and the best part is that those details not even meant to draw attention still were painstakingly created anyway, and there is nothing to suggest you got lazy anywhere in this.

Talk about dedication.

There is one thing though: the angle of this is rather bland, coming from you, as your gallery has several examples of Space Marine close-up paintings. You even have a highly-stylized rendition of Lorgar, Papa Word Bearer himself, from a similar distance, even if it had a different perspective.
There is also, of course, the fact that this is a Marine. Chaos or not, they're essentially the face of 40K... whether GW forces them to be or not, as the internet is covered several leagues over by images of Space Marines.

What this is, though - it is a glorious take on what appears to be one of your favorite types of portrait. I will probably continue to pause over the course of the day and come to stare at the details of this masterpiece for a long, long time, I think.

Great work.

So there. Not sure how much use this'll be to you after so long, but there are my thoughts.

Having watched your work for a year or more now, I must say that this may be one of the best work you've constructed so far. A piece does not have to be made up as an action scene to thrill the mind. There's so much detail in this piece that there doesn't seem to be an end to how long you can look at it and try to examine every little piece, without it getting annoying like many artists make.

Not to mention your practice at rendering texture and materials have really paid off, by the looks of it, from the rough, scarred texture of steel on the armour to the fleshy faces on the shoulderplate.

And finally, I am quite pleased with the way you've managed to show personally from the Marine with so simple a pose and apperance.

All in all, a fine work, nigh-perfect.