Yesterday we played some more All Zombies..., and it was another good session. You know you've got the tone of gritty survival horror right when your players are celebrating the discovery of six duck eggs hidden in a patch of reeds by a pond.
Less violent than previous sessions (there was only one combat encounter, though quite a big one), this one was all about the harshness of trying to survive in a post-apocalyptic Siberian wilderness: endurance saves against the cold and starvation; constant delays so as to undertake foraging and hunting for food; and taking pleasure in small mercies (like the aforementioned duck eggs). Even the few inhabitants of this place are hostile - invited to share the food and shelter of a mysterious cult of Old Believers, the players found themselves drugged by stew (made of human flesh) in order to be quietly murdered and eaten later. They were saved only by the fact that one of the players was sensible enough not to actually eat the stuff.
It was also the first session in which no zombies have appeared or been encountered. This has had a noticeable effect on the mindset of the players - the more rumours they hear about the things the more their suspicions and fears seem to grow, so that they're now far more keen to avoid zombies than the danger actually warrants (in terms of the mechanics). Indeed, encounters with shotgun or rifle wielding bandits have far more potential to result in death than would a zombie sighting at distance, yet the players are remarkably more sanguine about the former than the latter.
Less is more, and all that, but it doesn't hurt to remind onself as a DM that this is very much a truism.