Difficulty

There are four difficulty levels in Left 4 Dead. Each level refers to the difficulty of the campaign, and each has drastic differences over the next. For example, on the Easy difficulty, it's rare to see a horde outside of the occasional Crescendo Event, whereas on Expert it's likely you'll face a Horde around every corner. The specific differences between each difficulty level are listed below.

Easy
As the name suggests, this difficulty is the easiest of the four. Players receive a ridiculous overabundance of health packs, to the point where you can be completely reckless for the whole game and still have plenty of packs left over. Common infected die from a single pistol shot, meaning that a single player with a pistol can decimate a horde in moments. Shotgun shells kill infected at sniper range. This mode would be recommended for beginners and experienced players who simply want to relax. This is also a good (albeit somewhat cheap) way of getting some Achievements.

Details

 * The Tank has 3000 health points.
 * The Witch has 500 health points.
 * Friendly fire does not inflict damage, but friendly fire incidents will still register and disqualify players from earning the Safety First achievement.
 * Molotov cocktails and gas cans, once ignited, can still cause damage to other Survivors, so watch where you throw them.
 * Common Infected inflict 1 point of damage per hit, 0.5 if attacking a survivor's back.

Normal
Normal mode is fairly balanced, as far as the difficulties go. It's harder than easy, but not something that will quickly decimate an inexperienced player, such as in Advanced or Expert. However, The Director's evident sadism will begin to show more often here, especially if the players are doing well.

Normal is also the set difficulty for Versus mode.

Details

 * The Tank has 4000 health points.
 * The Witch has 1000 health points.
 * Friendly fire inflicts 10% damage per hit to the Survivors.
 * Common Infected inflict 2 damage per hit from the front, and 1 from the back.

Advanced
Advanced is for substantially more experienced players. Although it's not as devastating as Expert, it demands a high level of competency and care. If the player is not careful, then they should be ready for multiple tries per map, as things can get pretty hard—and friendly fire incidents are the bane of a serious player, as they take off quite a bit of health here. The fact that first aid kits don't spawn very often only serve to further exacerbate the problem. Advanced is the least played difficulty, mainly because no achievement stimulates the player to play advanced. Normal doesn't either, but is widely used by new players, which unfortunately, forces more experienced players to care for them.

Details

 * The Tank has 6000 health points.
 * The Witch has 1000 health points.
 * Non-headshots get no bonus damage
 * Common Infected inflict 5 damage per hit from the front, and 2.5 from the back.
 * Friendly fire inflicts 50% damage per hit to the Survivors.

Expert
Expert mode is, obviously, the toughest difficulty of the four. It is not at all recommended for the inexperienced or impatient, as it may take many tries for the player to complete a single chapter. In this mode, never fear death, and always be on the move. Standing still is an almost guaranteed way to get killed here. But then, even moving has its down sides. The player must be ready for anything, anticipate everything. However, they must remember that in the end, it is very likely that they will die. Though it can get incredibly frustrating at times, with practice and a great deal of patience, it is possible to get through.

There are many unique nuances in Expert difficulty that you won't see in the others.

The Horde
The time between Panic Events in Expert mode is a lot shorter than in the other modes, and the size of the Horde is larger. On top of this Boomers will constantly be sent at the Survivors as well, puking and exploding all over you. This makes progressing somewhat difficult as you may be stuck within one area for long periods of time attempting to deal with the infected.

Details
This is a mostly complete list, extracted in part from the exposed cvar interface. See Console commands for information on the cvars.
 * The main attack of the Witch is a one-hit kill that bypasses incapacitation. Also, if Witches are avoided without being startled the Director ensures to continually spawn them in your path, sometimes spawning three or four per level if the player(s) manage to keep avoiding them.
 * The Witch has 1000 health points.
 * The Tank has 8000 health points.
 * Fire inflicts more damage to the Survivors.
 * Friendly fire inflicts full damage to the Survivors. AVOID FRIENDLY FIRE AT ALL COSTS. One attack from an auto shotgun will instantly incapacitate a teammate.
 * Common Infected inflict 20 damage per hit from the front, and 10 from the back.
 * First-aid kits are never found on Expert outside of the Safe and before the Finale. They are instead replaced by pain pills. Pain Pills are surprisingly common.
 * Minimum interval between Horde attacks is much shorter.
 * Hunter melee attacks deal 40 damage, and they deal much more damage when a survivor is pounced.
 * At close range the Tank is extremely good at cutting off survivors who try and turn, even a little (By turning I mean any movement other than running directly away from him.) The only way to outrun a Tank on Expert at close range is to continue to run directly away from him. The same goes for his Rock throwing attack. The best way to avoid his Rock throw attack is to continue to move in one direction while he prepares to throw the rock, then when he throws it change directions quickly. Or simply remain still and move at the right time.
 * The Boomer appears to have increased range with his vomit attack, getting up to the point of almost being able to "snipe" you with it.
 * Enemy reactions are much faster; for example, Smokers will often pause for several seconds on easier difficulties before snaring you with his tongue; however, on Expert, they won't hesitate to do this.
 * The worse Special Infected will almost always spawn on the path(s) you must take, namely the Tank and the Witch. However, observant players might notice a roaring, muscled or hunched, crying mass in the distance, and closed captions can help predict enemy locations, which can help a player get the drop on the Special Infected.