Francis and Zoey were originally going to have an interest in each other, but the idea was scrapped due to complaints that it was too distracting.
Zoey's voice actor during the beta stages of the game was slightly different compared to the current voice actor (beta Zoey has a somewhat deeper voice compared to the current Zoey voice). The sound files for the beta voice actor are still used if Zoey triggers the start of a finale. Zoey's screams when she falls to her death are also from the voice actor in the beta stages, but this one was intentionally kept in the game because Valve liked how well the screams sounded.
Francis originally had a different look (along with the rest of the cast) that showed him to be overweight. Hilariously in the final version of the game, Louis will still sometimes call Francis fat if he gets shot by him, even though Francis is currently muscular.
During the No Mercy campaign, Bill was supposed to express paranoia about becoming an infected, but this was reduced to Bill asking his fellow survivors to shoot him should he ever turn.
Pipe Bombs did not have the smoke alarm attached to it at first, so it was a standard grenade. Testing proved the use of the grenade was too difficult since infected tend to not stay in place when they chase you, so to make it useful, they tried making it instantly kill Tanks, but even then they decided that it still wasn't very enjoyable to use. They added the attraction feature on the bomb to make the zombies go after it, and make it more fun to use.
Valve originally planned the game to be restarted from the very first map if all the players died. This was changed to have the survivors restart on the map they died in instead since it was simply too much to lose all that progress from the past 40 minutes. The idea was revisited in Left 4 Dead 2 in the Iron Man mutation mode.
Alongside this idea, the campaigns were originally all meant to consist of one map each, rather than three to five. The levels were eventually split up, with saferooms added to work as one-way transitions between them.
Similarly, survivors that died stayed dead until the next map. Valve decided that dead players waiting over 10 minutes to play again was just too much and they brainstormed the idea of rescue closets where, in logic, a survivor that was too injured barricaded themselves in a closet but are now trapped. Not only does this cover the problem of players rejoining quickly, but also mimics the zombie movies where survivors find other survivors trapped in a room.
The helicopter pilot from the No Mercy campaign was supposed to become an infected after he rescued you, causing the helicopter to crash and start the events of the next campaign. Play testers found this concept too depressing since they felt all the fighting the survivors did to get saved was a total waste, so they made each campaign their own story. This idea has been revisited in the sequel where the campaigns are connected to each other and Valve also retconned the L4D1 campaigns to be connected in the story.
Survivors were able to lock doors to slow down the infected, but the idea got scrapped for making the game too easy.
Left 4 Dead 2 originally had melee weapons break, but it was scrapped for being too annoying. Similarly, the wooden walkways in the Swamp Fever campaign could break and force the survivors to wade through the water, but it was also found to be too annoying and cut.
Melee weapons could also be dual wielded, but the idea was cut after tests showed the idea could not be balanced.
Internal development notes released alongside the "original" L4D1 Survivor models reveal that at least a baseball bat, as was preorder-DLC-turned-free-content in the second game, was to be part of the original game.
Left 4 Dead 2 was supposed to have an ammo pack, an item that a player could deploy where the team got more ammo. This idea clashed with Valve's idea of "less ammo piles for more guns" and play testers saw the ammo pack as worthless since it replaces the first aid kit, so they scrapped the ammo pack concept and replaced with the Incendiary/Explosive packs.
Similarly, fire and explosive packs were to be generated by the AI Director randomly which survivors could get an infinite supply from, but play testers were reluctant to leave the area since they wanted to keep the special bullets. This idea was changed to being a deployed ammo pack that survivors could place at any time and be used by all survivors once each.
Laser sights were planned for L4D1 but got scrapped due to time constraints, but some modded servers have the lasers enabled. Lasers reappeared in L4D2.
Bile Bombs originally were able to splash other players, creating the same effect as being hit by a Boomer's vomit, but this feature got disabled for, you guessed it, being too annoying in the heat of a major fight.
VS mode originally allowed players to take control of a common infected while they wait to respawn as a special infected, but this was cut due to how weak common infected were in general and would just frustrate players.
First Aid Kits originally could be passed to another player like pills could, but this was removed since it was just easier to use the kit on a wounded survivor than to give it to them for them to use on themselves.
When a survivor was incapacitated, they had the ability to crawl so they weren't stuck in one place. This idea got axed for allowing survivors to negate any damage done to them by being able to move around and crawl to their teammates for help. Some modded servers have this feature enabled.
Smokers and Hunters originally had the ability let go of their victims to attack a different survivor. The concept was cut out due to it making the Smoker and Hunter too overpowered (in this scenario, Smokers and Hunters could incapacitate a survivor and let them go to quickly take down another survivor) and it made getting their achievements too easy. In a similar fashion, Boomers originally had the ability to explode if they fell from a great height, effectively going kamikaze on survivors and splashing them in bile before they could react. AI controlled Boomers in campaign mode can still do this (usually unintentionally) but this feature is disabled in VS mode for obvious reasons.
Boomers originally did not have the horde summoning ability and it was essentially a walking undead bomb whose explosion dealt major damage to anyone that was too close when it popped after being shot. At the same time, another infected, called the Screamer, was being tested where it could not attack survivors but if spotted, it would run away and let out a scream that called a horde. When Valve scrapped the Screamer, it merged the horde calling ability to the Boomer and scrapped the explosive damage idea for it. Traces of the explosive damage can still be seen as the Boomer still destroys doors and windows nearby when it dies, as well as pushing back survivors who are too close, but the explosion deals no actual damage. The programming for the Screamer running away would later be recycled for the Fallen Survivor Uncommon Infected.
Smokers got the name originally because they would appear in a puff of smoke, abduct a survivor, and disappear. This however proved to be too difficult for players to use and was instead replaced with the tongue as the Smoker's primary attack. His smoke effect also went through several iterations, where it would instead be a secondary attack that pushed people back, There was also a variable for it to causing poison damage (leftover of this can still be seen in the game, where survivors cough as they go through the smoke) but these were all ultimately cut (the smoke now obscures your view, but does nothing else to you). Another feature was that those being caught by smokers could also shoot back, freeing themselves. This still remained in the game in a small capacity; you had about 2 seconds after the smoker hit you to shoot him, but after that window passes you'll be helpless.
The Witch was originally supposed to attack the entire group if disturbed. However playtesters could not consistently kill her before she downed every member of the team and she would often spawn in a choke point, making the level impossible to finish. This was deemed too hard and she was restricted to killing only one player, so if the team was down the man with the lowest health could, in the worse case scenario, sacrifice himself for the other three.
The Spitter, Jockey, and Charger had several design changes as Left 4 Dead 2 was being developed, but the Jockey and Spitter stand out for several reasons:
The Spitter's first design had it in a nightgown which was scrapped because Valve had difficulty trying to render a free flowing piece of clothing. The next design made it look like a female Boomer in a sense, but was also scrapped. The design after that made the Spitter look like a zombified, pregnant Pippi Longstocking, which Valve quickly scrapped when play testers made pregnancy jokes, until they came up with the final design of the Spitter. The Spitter's spit originally stuck to a survivor that got hit and it would damage them over time, but it was cut out for being too overpowered. The acid pool from the Spitter also changed colors to reflect its intensity but it was then changed to be just green.
The Jockey's special ability went through several iterations. The first design had the Jockey grab both of the survivor's arms and restrict them. The second design had the Jockey completely block the survivor's vision, which would also blind the player being ridden on. The final version of the Jockey's attack has it clawing at the survivor's face while leaning to force the survivor to move in that direction.
The Charger's original charge attack only bowled through the survivors, while its melee attack was what allowed it to grab a Survivor and pound them into the ground. They changed it so that the Charger grabs the first Survivor it hits, and simply bowls through the rest.
Initially the Fallen Survivor was cut due to not being thematically tied to any campaign and was basically a mobile gascan (he had molotovs on him that would trigger if shot). This made any hordes they were in too easy, as killing one would set the entire horde on fire. He was later re-released with The Passing with new programming causing him to run away, but also spawning with droppable supplies such as Med Kits, pills and grenades.
The Sewers of No Mercy were originally supposed to have the players go through a gas station. This was later cut because the gas station, as part of a set piece, could be destroyed and blew up spectacularly. However this often incapacitated the entire team as it was also tied to a crescendo event, and in the confusion the players often shot at the pumps, blowing themselves up.
The first map of The Parish campaign for Left 4 Dead 2 was originally supposed to be a lot longer. Instead of the map ending in the convenience store, it would continue on through the park and the bus parking lot. However, the length of the map was simply too long and it was cut down into two maps instead. You can actually access this beta map on the PC version by entering in the console "map c5m1_waterfront_sndscape" (without quotation marks).
The finale of Dead Center was originally suppose to have the players chase down Jimmy Gibbs to get his car keys. Much like the screamer above, Gibbs turned out to be too hard to find among the constant-spawning hordes and his higher HP made him hard to kill once found. Gibbs can still spawn during the finale, but as an incredibly rare Easter Egg, while the finale itself was retooled to showcase the new Scavenge Mechanic.
Before Left 4 Dead started development Valve worked on a fantasy game involving Fairies. It wasn't very good so it was scrapped. Valve thought some sort of co-op game would be better instead which eventually led to Left 4 Dead.
For the Left 4 Dead 2 survivors, Nick was originally created as a convict who had broken out of jail while still wearing his orange jumpsuit and then stole a white suit later on. The character concept was scrapped and changed to Nick being a con man in a nice suit. In a strange twist, Nick was the only survivor whose initial model was unchanged throughout development (the other 3 had some changes to their looks).
Nick's suit in the beta was originally crisp white; dirt and grime were added shortly before release.
Rochelle had several design changes to her appearance during development. Originally, she wore an orange shirt and had her hair in a bun. Rochelle's next design changed her shirt to an orange-red color with the Depeche Mode band logo on it and her hair changed to a ponytail. The final design Valve settled on was Rochelle with cornrows and her shirt color changed to pink.
Coach went through several clothing colour iterations but otherwise his model more or less remained the same. Early concept art revealed he sported a white and light blue shirt (along with a gridiron helmet and whistle, though his reveal in the first E3 2009 trailer omitted those), and then opted for an aqua and gold shirt with a puma logo instead. His final design eventually came to be the purple and gold shirt with 'FHS'.
Ellis' early designs were initially a typical mechanic look - green hat, completely white shirt and light blue overalls. His E3 reveal changed this to orange overalls, a white and blue shirt and his trademark 'blue truck' cap. This was then changed to his beige Bullshifters t-shirt and back to the light blue overalls upon release.
Several of the levels in Left 4 Dead 2 had some concepts shown in the trailer that are quite different to the final versions of the game. Dark Carnival's coaster level was going to have some sort of wreckage that the survivors had to clear out which would cause an alarm to go off and alert the zombies. The concept changed to the survivors having to activate the ride and run the track to shut the alarm off. Likewise, the finale in The Parish would have the army bombing the bridge as the survivors crossed it, but the idea changed to the army only bombing the bridge after all the survivors reach the helicopter.
The Hard Rain campaign had some level designs scrapped in the final version. Originally, the Burger Tank was farther away from the start point, which meant players had a ways to go before they could pick up weapons. Players also had to turn on the lights for every house they passed through since by the time they made the return trip, the hurricane made everything dark. Valve moved the Burger Tank closer to the river and they also made the lights from the houses turn on by themselves.
The CEDA/Hazmat Infected were originally going to show up in The Parish while the Riot Infected would show up in Dead Center. Valve decided to have them switch places.
An infected called the Leaker was going to be in Left 4 Dead 2 and while no one really got to see what it looked like, the Leaker in the game would plant itself in the ground and then ooze out some sort of acid when the survivors got near. The Leaker got cut due to it being too difficult to use effectively and it was basically doing the same acid attack as the Spitter.
The Riot Shield in Left 4 Dead 2 was a melee weapon players could use, but Valve removed the spawn script for it so it would never appear. Mods can enable the weapon to be usable again.
During the development of The Passing campaign, Valve's original plan was one of the Left 4 Dead 1 survivors would be dead, but the unlucky victim was picked at random for every play through. The idea was hinted further when Valve posted a teaser poster of The Passing on their web site and the old survivors in the background would randomly switch characters among a group of three whenever the viewer reloaded the page or revisit it later. Due to difficulties with getting Jim French (Bill's voice actor), Valve decided to make Bill be the dead survivor at all times.
There was originally going to be DLC that was based on The Cabin in the Woods, with both the cabin and the Facility. This can be seen in the film itself, where, in the Facility, the four main Special Infected can be seen.