Throughout the series the sound effect of a gun being cocked is used incorrectly. Often when a character draws a pistol the sound of a hammer being cocked is heard, but often the gun being used does not have an external hammer, but a internal hammer known as a striker plate (like a Glock). Or even with guns that have external hammers, like revolvers, we hear them being cocked but when we see the gun the hammer is still clearly uncocked.
In the first two episode of the first season walkers are depicted using tools. In the first episode when Rick is in the gas station he sees a little dead girl who is walking and she bends over and picks up a dull.
In the second episode one of the walkers trying to get into the store is using a big rock to break through the glass door.
The zombies' organs still work, as shown when Rick and Daryl cut open the walker looking for Sophia. Blood would need to be pumping through the veins in order for this to happen. Blood pumping would not allow the zombies to decay.
The fuel they are using in their vehicles if it was an ethanol blend, would have degraded significantly in just 12 months, and this show takes place over a number of years, by which time the fuel would be unusable, and yet they seem to be using it without issue.
There is no clear reason why Michonne's technique of walking with two zombies on leashes deters other zombies from attacking her. By this logic, anyone walking around amongst the zombies, regardless of whether or not they are covered in zombie blood and guts, should be safe, so long as they don't do anything to draw attention to themselves.