One of Left 4 Dead’s features that greatly add to the replayability is the use of both a selectable difficulty AND an on-the-fly skill based difficulty generator in the form of the AI director. Because of this, the game not only changes to fit your skill, but you can set a general level in advance so that the game doesn’t murder you before you have a chance to have it go easy on you. So while each level has varying difficulty, each of the difficulties is, at its easiest, still harder than the last at its hardest.
Easy mode is basically “Baby’s first campaign”. The infected are so ineffectual and the special infected so slow to kill you that even the newest of players can smash through, and is therefore mostly only used for achievement farming (such as Stand Tall, Unbreakable, Akimbo Assassin, etc).
Normal mode is a decent challenge, but most people will get through it without much problem. Basically what it says on the tin.
Advanced mode is where the challenge starts to set it. You’ll probably have to replay a few maps a few times over, especially the finale, and get used to limping towards a safehouse with your colours missing. But still, a decent team of players can get through in a reasonable time.
Then there’s expert.
Oh, expert.
It used to be back in the day that you could switch difficulties for versus. This was then standardized, presumably because someone noticed that playing versus on expert is akin to taking away a player’s gun and spawning in a tank.

Yep, that’s a totally fair fight.
Let me try to better articulate why expert is a sudden, painful leap in the difficulty curve of the other difficulties into sheer frustration.
You know common infected, those delightful little weaklings who charge into a shotgun and find themselves decorating the walls? On easy they do one measly damage to a survivor, and only half that if attacking from behind. On normal, that goes up to two and one. Still weak stuff. On advanced, they almost become a threat as they go up to five and two point five, but seriously? Five? You can take plenty of those hits. On expert? They do twenty. Damage. Per. Hit. That means that five infected you didn’t catch in time and you’re down on the ground.

Enough to incap you instantly. Thanks expert.
But you’re probably thinking, so what? It’s normal infected. They go down easy. Sure, they pack a punch down, but, hey, I’ll just be a bit trigger happy.
About that? Don’t.
Friendly fire is always on in Left 4 Dead, but it’s toned down. You only do 10% of the weapon’s normal damage on normal, 50% on advanced. On easy, it doesn’t even do real damage, just yells at you. On expert, this magic shield disappears and you do full damage. Just for reference, the shotgun incaps in one shot at medium range now.

This man is going to try and shoot a 20-damage-per-hit zombie away from you and incap you instantly.
There’s plenty more. The witch KILLS (yes, not incaps) you in a single hit. The Tank does 150 in one hit- that’s enough to incap you in one, then kill you very, very fast. Hunters do 40 damage per hit, and 15 per second when pouncing. And just for more fun, with a couple of location exceptions First Aid kits NEVER spawn anywhere except the safe room and before the finale.
Now, some of you are no doubt saying “Why, you foolish person, this is of course to make it a challenge, and I laugh at your weakness!”. While it’s true that expert SHOULD be hard, the immense gap between it and advanced is a low blow, and far, FAR greater than the gap between any of the other levels.
So here’s a tip, reader- expert actually does mean expert, for once.
