It seems to me that worlds and environments are the biggest challenges in modeling. Because when doing them, they can seem to go on forever, and it is easy to skip on the details, which, when too many details are lost, can ruin the whole thing. This peice of work is awesome in all its detail and anyone, even a new guy like me, can plainly see the huge amount of work that went into it. I really like it, makes me wanna shoot some bad guys!