The word "realism" in the context of a fantastical setting such as that of a zombie apocalypse may seem a little bit out-of-place, but No More Room in Hell is to Left 4 Dead as Insurgency is to Call of Duty. The overall concept may be similar to other zombie apocalypse co-op adventure games, but the mechanics favor a level of real-life authenticity that others lack, and it's much stronger for it.
One of the biggest differences is how rare guns and ammunition are. Finding an M-16 with a whole entire full magazine is a treasure to be coveted and a tool not to be wasted. Much more common are six-shooters, 9mm handguns, lead pipes, fire axes, and more of their ilk.
Ammunition comes in different calibers that must be matched to the appropriate weapon. 5.56×45mm NATO cartridges used in the M-16 won't fit into a .357 revolver, and .22s useful for a Ruger rifle will be rather useless when you only have a double-barrel hunting shotgun instead. It's paramount that the players share ammunition with each other to keep everyone armed, especially when someone finds ammo they can't use but know someone else can.
Melee weapons, which come in a wide variety in levels of effectiveness and inventory weight (more on this later), will be the real primary weapon for dealing with most zombies, especially when there are many players in a game (resulting in a much thinner distribution of ammo; unlike Left 4 Dead, more than four players can play together, depending on server settings). While melee weapons do indeed follow the zombie apocalypse adage "swords don't need reloading", using them comes with a significant risk: not only can players not see their own health levels and take no insignificant amount of damage with hit, and not only will zombies sometimes grapple onto victims and stop them from moving, but players run the risk of being bitten and infected.
Infection cannot be cured; only temporarily staved off with pills. Infected players can ultimately complete levels together with non-infected players, but without a steady supply of pills, they will eventually drop dead and their corpse reanimate as a fast zombie. As infection worsens, the player's vision will gradually become more obscured, until they are eventually completely blind. If friendly fire is enabled in the game, this would generally be a good time for another player to put the victim out of their misery before they have to deal with another zombie.
Players can also start bleeding out after being hit. Their health levels will slowly deteriorate over time until a bandage is applied, adding another important tool to keep an eye out for. When a player’s health becomes critically low, their vision will turn to black and white. This is the player’s urgent warning: find a medkit, find a bandage, find something or you’re toast.
There are a multitude of tools and items to be found in No More Room in Hell. Pills, medkits, grenades, guns, melee weapons, and gas cans all make an appearance in identical or similar roles to their Left 4 Dead 2 counterparts, but added are flashlights, radios (players can only hear each others' microphones if they are near each other unless they are carrying radios), welding torches, bandages, individual ammunition boxes/half-spent magazines, and more. Every map is peppered with item spawns, but the sheer variety of them can make it difficult to find the specific one that's actually needed.
It's entirely reasonable to leave an item pickup where it is because it might not be worth carrying in your inventory at the moment. Inventories are separated into two sections: guns/items, and ammunition. Both work on the same system - the item's size and weight affects how much space it takes up in your inventory. The closer your inventory is to being full, the slower the player will run. Carrying around both a sniper rifle and a pump-action shotgun may be a powerful combination, but it will also leave very little room for other equipment. It might be wiser to give one of the weapons and its accompanying ammo to a teammate, or even to simply leave one of them behind.
The zombies themselves come in the slow, shambling variety, many of whom will ignore the player if they don’t linger around for too long. They can soak up many bullets to the chest, but will crumple quickly with one or two shots to the temple. Occasionally, a fast runner - or even worse, a half-height running child - will appear and chase down the players; these enemies especially dangerous not just because they can attack the player faster, but also because they are harder to hit while being just as durable as a normal zombie. This forces the players to make a split-second decision: risk infection by trying to hit it with a melee weapon, or burn through large amounts of precious ammunition to gun it down before it gets close.
Levels come in two modes: objective and survival. Objective mode is analogous to Left 4 Dead's main adventure mode. Players must work together to travel from point A to point B, scavenging for supplies and overcoming obstacles along the way. Routes and obstacles, in a similar manner to Left 4 Dead’s system, vary randomly. However, unlike in Left 4 Dead, the different routes can sometimes be dramatically different from the others, and some levels even can be played backwards. This adds a great amount of replayability to the official levels, and there are many quality player-made maps floating about, as well.
Survival mode tasks the players with not only simply surviving successive waves of zombies, but also with defending certain zones on the map from zombies. Players will receive a supply drop at a random spot between waves containing random weapons and items, which must be found and used in enough time to return to the holdout area before the next horde of shambling monsters arrives. Both surviving every wave and protecting at least one of the zones will achieve victory, but failing in either will result in a defeat. The quality in survival maps varies; some are significantly more fun to play than others, but there are, of course, many custom maps for this mode as well. Simply put, it’s an interesting alternative to the Objective mode that some players might hate and some might prefer.
The game definitely has that distinct “Half-Life 2 mod” feel to it (which is to be expected given that that’s what it started as), so don’t expect any revolutionary graphics or animations. The gameplay is the important aspect here, though; No More Room in Hell is an excellent example of the “graphics alone don’t make a good game” argument. That said, the age of the Half-Life 2 engine does come with some limitations, namely in the netcode: a high ping to the server can make zombies significantly more difficult to hit, especially given how important headshots are.
tl;dr: No More Room in Hell is a unique take on the co-op zombie survival adventure game that puts a slower, more realistic spin on Left 4 Dead’s formula. It demands that players work together and share resources if they hope to survive, but that makes the grand achievement of simply completing the level all the more rewarding. Being a standalone version of what used to be a mod, the game is free (not free-to-play-with-strings-attached; just free), so you have nothing to lose by trying it out.