There's only one thing that bugs me Or maybe it's three things.
1a) Given what we are told of the nature of the disease that turns people into "walkers" (zombies), it's completely, absurdly, impossible that a zombie apocalypse has taken place.
1b) This gargantuan lapse in the logic of the back story is circumvented by the lamest possible plot device: having the P.O.V. character be comatose when it happened.
1c) Wait, did I say "lamest possible?" Make that "second lamest." Let's also make the character so incurious that he never asks anyone, not even a doctor at the CDC, how things got so bad while he was unconscious.
The living outnumber the newly dead by a factor of about 25,000 to 1. If the zombie illness can only be contracted by contact with a zombie (as in, his teeth in your flesh) or his blood, and if the zombies are brainless and slow-moving, how did they ever get the upper hand? Such an illness would have engendered not apocalypse but mild and embarrassing social inconvenience.
The only way to get a zombie apocalypse is to make the zombifying illness wildly virulent in the conventional, airborne manner, but there's been no hint that that's the case. And for good reason: if that is the case, the big terror is just about catching something rather worse than the 1918 flu, and the whole bit about avoiding the resulting zombies is just a footnote to the general epidemic story. Everyone would be most paranoid about other apparently healthy (but secretly infected) humans, not about the zombies--whom the living disease carriers would outnumber 10,000 to 1.
This problem could have been solved by having a terribly virulent zombifying disease that for some unknown reason ceased to be virulent (e.g., universal adaptation by human handwave lymphocytes), leaving only the zombie vector to spread it after the majority of the world had died. And maybe I can pretend that that's what's going on here, even though no one has mentioned it and no one seems to have been affected psychologically by the period of infection paranoia (which would have significantly deepened the human drama, given that they all have to trust one another now). But it's kind of scandalous that the basic premise of the show (and I presume the comic book) has been so shoddily thought through.