I'll let the cat out on this. Zombies can get aggro when hit and nearly run now. It can go from slow to a brisk walk to nearly sprinting. It makes combat a lot scarier because they can suddenly sort of lunge at you. Hitting them again seems to sometimes slow them back down, but it is very cool. Sorry if your an always walk guy. I mean they don't just turn feral if thats what you reading here, mostly its a small increase in speed, just enough to make you feel a little uneasy and cause them to close the gap and hit you a bit more.
On the plus side, (IMO both are a huge plus, zombies were zero threat before and I didn't need armor at all) players now have a glancing blow so as you swipe your sword across a guy it might hit his arm and then hit his face for the intended hit. If you miss by a milimeter you will get a glancing blow, so less missed head shots that result in nothing at all, and it kind of helps make combat feel less binary.