1. Zombies trapped in body bags: They cannot bite, scratch or cause any kind of direct harm, but as they are squirming around in there, there is a chance they will knock you down if you get close to them. Maybe the squirming behavior can be triggered if they hear noise with a eventual calm down after some time has passed. Upon opening the bag, a zombie corpse will spawn on top of the body bag.
2. Zombies with missing limbs: Zombies without a arm have a slightly higher chance of causing a laceration wound do to the fact that the broken arm bone is sharply pointing out of it's shoulder. This can be something like the medium and short ranged melee weapons the player uses, but for zombies. Also legless zombies crawl.
3. I don't know if this is implemented or not, but it seems to me like zombies that wear helmets are just as easy to kill as those that don't. Having zombies be more protected by the clothes they wear would be more realistic and we can have tough zombies in riot gear for example.
4. Barbed wire zombies. The zombie got entangled in barbed wire or some other spiky/sharp object. This zombie has a higher chance of giving you a scratch, laceration or some other wound which does not lead to getting infected with the virus.
EDIT: Added a few more.
5. Heavily decayed. These zombies have been out in the world for some time, might have been the first people to actually get infected or had the misfortune of being thoroughly ripped apart by a large group before they turned, in any case, their appearance makes the player nauseous.
6. Known zombies. Sometimes you might just cross paths with zombified loved ones and this causes you to have severe depression upon seeing their shambling husks(Don't know if this will be a thing when the NPC update comes out, so here you go.).