Matthew and Thantsants have this year both expanded upon the subject with looks into influences upon the Citadel Orc and personal journeys through fantasy art respectively.
I feel compelled to add my pennies worth to the debate but much of the ground I would cover has already been covered quite admirably. I do see (or rather do not see) one glaringly absent figure though the subject has been lightly touched upon.
I believe the figure to be amongst the most influential on the young minds of our generation in determining what we will and will not accept as a proper orc. By the time it was produced the Citadel orc was already well evolved but I'm be suprised to hear if any other orc figure ever made it into as many homes. Figured it out yet?
Rather than post the picture here is a link to a picture so you can have a good unbiased think and then see if you agree.