I am not trying to say that it isn't like aggro. There are some superficial similarities between the marking system and aggro. However, they are only superficial. What with combats no longer being a party of PCs vs one or two monsters and now involving much arger groups of monsters and having a lot more mobility involved, making the defenders "sticky" adds an important variable to the combat. I think that the important distinction between the two systems is that in D&D, the DM controls who the monster attacks. The DM can ignore the marking penalty if the monster is one that would be able to make the tactical decision.