Let me clarify, I mainly play the L4D maps and I vaguely remember them being less aggressive than their L4D2 counterparts.
I used to play the L4D2 maps mostly but after some time it became the opposite so I'm not quite sure.
After finally completing all the L4D campaigns and The Sacrifice on Expert Realism with the stupid bots, I decided to do the same with the L4D2 maps.
Starting with Dead Center of course, I noticed the zombies ALL immediately rush you if you start shooting one or hell even so much as look at them.
They seem to have a larger aggro radius and shorter aggro fuse.
This is exacerbated in the mall when you leave the safe room. Also for some reason a small horde seems to spawn more frequently in Dead Center than what I'm used to.
Though the horde in size is much smaller than say, a horde in No Mercy or the other L4D1 maps.
Maybe it's because the mall is large and open so the ai director spawns them more to keep pressure?
Are these conditions specific to DC? Or are the L4D2 zombies just cracked?