Mine are as follows:
3 successes needed "of any value" to perform a "called shot" to a zombie's head.
If any of your players are zombie genre buffs, they will immediately state that they're aiming for the head of the zombie. If this is the case, I've ruled in the past that they gain 1 positive die for aiming, but that they must gain 3 uncancelled successes (of any value). If they do this, the zombie is down instantly; if they don't, they miss, or glance off of the head with their powered-up-but-inaccurate melee swing.
Why 3 successes? Because it's hard to aim at the brain of a moving zombie with a weapon, and all but true marksmen are unlikely to manage it on a regular basis. And for a melee weapon it's hard to utterly destroy the brain: edged weapons need to be swung/jabbed with enough force to penetrate the skull, and then have to be yanked free (which is hard due to the "suction" caused by the blood swell); whilst blunt weapons need to batter through skull AND mulch the brain to a degree where the undead would just... stop.
And mechanically? Lowering the requirement to 2 successes brings it so close to the "2 matching, uncancelled successes is a headshot" rule in the book that there's no inherent risk in spending time firing off bullets at such a precise target.
2 "matched" successes results in a "lucky" hit to the zombie's head.
The above rule is mentioned in the book, and is kept regardless of what zombies I use. This represents a "fluke" shot, whereas a targeted attack on the brain requires 3 successes of any kind.
A Feature can be both positive and negative. It counts as two Features.
Taken into effect after a talk with my GM-to-be and after reading the "Wrath" forums, this house rule allows a character to take a Feature that is both positive and negative, depending on circumstances. For example, someone who is +/-Overweight might gain a positive die when using their weight to keep a door pinned, but would take a negative when trying to run distance. Another example is +/-Small Size: someone who is small (a young character) might be at a disadvantage in a grapple, but would gain a positive die when trying to navigate confined spaces, or keep hidden somewhere that is cluttered and cramped.
When creating a character, the player can choose to perform ONE of the Step 2 changes ONCE PER CATEGORY.
As I play primarily online, it seems a little off to allow strangers to vote on whether you're being truthful about your characteristic arrangements. So, instead, we skip that part, and assign Features as normal. Then, the player has the option to either increase or decrease a characteristic ONCE PER CATEGORY in order to gain/eliminate additional Features, as per Step 2 in the book. This keeps control of the character's design firmly in the hands of the player, but remains largely in-keeping with the rules.
When a zombie inflicts Physical stress, immediately roll one die. On a result of 1-2, explain that the attack results in a bite, and that the next Trauma gained will be given the "Bite" descriptor.
I understand that Stress taken can be described vaguely in combat, and then the "true effects" of your hardship are discovered when you get a chance to analyse yourself during downtime (ie. when you convert Stress to Trauma); however, when it came to an infectious zombie bite, it never made sense to me to wait until that same downtime to determine whether you'd been bitten or not - I mean, given how terrified everyone is of being bitten, it'd be pretty obvious to you. So, instead, I make the d6 roll IMMEDIATELY when Stress is taken, and either describe the Stress taking as normal (on a roll of 3-6) or detail the zombie biting down (on a roll of 1-2). The player is then told that when his Stress is converted, it must become a "Bite" Trauma.