One word: Recon. Level design actually makes an entire player class worthless in the vast majority of the game. Let me explain.
All classes have two passive abilities which are constantly on, each can have one of three abilities chosen for a mission, which are only active some of the time for that mission. Let's talk about passives. Bertha/Medic has the ability to spawn with a first aid spray and the ability to heal more from healing items, which is useful throughout the game. Lupo/Assault has resistance to bullets, which is useful in much of the game, and the ability to reload faster, which is useful throughout the game. Beltway/Demolitions has resistance to explosions/ability to throw explosives at enemies more often, which are both useful fairly often. Four Eyes/Scientist has the ability to spawn with an anti viral spray, which is consistently useful, and zombie vision, which is useless. Spectre/Surviellance has a minimap upgrade which is fairly useful, and the ability to know if someone is aiming at him, which is very useful, with an item location ability I haven't bought or used yet.
Vector/Recon has the ability to walk and run silently for his two passives. Both are only useful if you are disguised/invisible(his two most thematic active powers), since enemies will shoot you if they can see you, and you generally can't flank enemies- most of the time there is only one route to your enemies, and they can see you as you take it. Moreover, being shot will cancel out invisibilty/disguise. And enemies will shoot at you as you move toward them, even if they can't see you, simply by trying to shoot your allies who are behind you.
This means that Vector/Recon can't flank enemies because of the level design, he has to run towards his enemies through a narrow corridor that has bullets flying through it, and getting hit by any bullets will cancel out his abilities, rendering both his passives and active disguise/invisbility worthless. This means that playing as recon, in either singleplayer or coop, just results in getting shot and killed over and over again.
The only way Recon would be viable would be if there were multiple paths to the enemy, and an invisible/silent person could creep along to sneak up on the enemy. Instead, there is often only one way to your enemies, which your enemies fill with bullets and explosions in order to hit your teammates, which cancels out your invisibility and renders both your actives and your passives worthless. There are just a handful of areas where you can take a roundabout path to sneak up on your enemies.
Every class but Recon is useful throughout the game. Recon/Vector is only useful occasionally, and it is my favorite class conceptually. Through level design, Slantsix made my favorite class almost worthless. Also, Lickers. Lickers are blind, and rely on hearing. But they can apparently "see" Vector no matter where he goes, will attack him as he runs past, even if he never shoots a gun or does anything, even though his passives supposedly make him silent.
Also, allied NPC's are idiots. They will literally run from a safe, defensible position from where we can safely shoot zombies, run in front of my line of fire right up to the zombies to punch them, and then complain about getting infected/shot. They also refuse to use anti viral sprays to cure those same infections, forcing me to either waste mine to cure them or to shoot them in the head and then revive them, and they refuse to revive each other. That said, it is a surprisingly good game despite it's flaws.
