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 recieve 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 1 point of damage per hit to the survivors.
 * Common Infected inflict 2 damage per hit from the front, and 1 from the back.

Advanced
Advanced is, of course, for slightly more experienced players, though it's not as devastating as Expert. If the player is not careful, 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.

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.

AI Survivors
The AI Survivors are not as effective at fighting the Horde as they are in other modes, perhaps to make it so that the player will not rely on them too heavily. They also have a hard time keeping up with the human players, possibly forcing you to leave them behind. In this case alone, it's understandable to sacrifice one of them, especially if it's for the greater good of those who make it.

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.
 * The Tank takes longer to burn when set on fire.
 * Fire inflicts more damage to the Survivors.
 * Friendly fire inflicts full damage to the Survivors. AVOID FRIENDLY FIRE AT ALL COSTS.
 * Common Infected inflict 20 damage per hit from the front, and 10 from the back.
 * First-aid kits are replaced by pain pills. Pain Pills are suprisingly common.
 * It is possible to find First-aid kits outside of Safe rooms and finales, but it is incredibly rare.
 * Minimum interval between Horde attacks is much shorter.
 * Hunter attacks deal 40 damage.
 * The Tank runs faster than the Survivors. In addition to their increased speed, Tank attacks are now instant incapacitations and their accuracy with the rock throw is heavily increased, even predicting which direction a human survivor will run in.
 * Special Infected, namely the Boomer, Smoker, and Hunter, are almost always spawned in conjunction with one another, causing seemingly coordinated attacks which can devastate an unprepared team.
 * On rare occasions, Hunters will spawn near Witches and continually claw them, causing either the AI controlled Survivors to try and shoot them while at the same time startle the provoked Witch or causing unaware players to startle the Witch much easier than normal. Occasionally, though more rarely, Boomers and Smokers will also attempt this, but they have the same effect as the Hunter.
 * 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.