First off, the Sandman, introduced in the Scout update. This unlockable replacement for the bat tosses a baseball that stuns the foe (and until an update several months after its release, even in ‹berCharged state) for up to 8 seconds (the farther away you launch the ball from, the longer the stun duration). And after stunning someone, you can pick up your ball (or someone else's) and use it again. It became the first weapon in the game to be banned in league play. In fact, the Sandman caused so much controversy that Valve has nerfed, then buffed, then nerfed it numerous times. Seriously, look at how many patches have been applied to it. In its current state, it sees relatively little use in all types of gameplay, since few players are able to aim the stun balls accurately enough for it to be worth the health loss.
How about Natascha? In theory, it was supposed to be weaker because it did less damage per bullet than Sasha, and its "slowdown" effect was meant to make it a support weapon that made it easier for teammates to hit people you were shooting at (or for you to land more bullets on them). In practice, the slowdown effectively become tractor beams, making anyone that been shot ONCE, regardless of distance, become a statue, allow even more shots to be landed. Thus, the second weapon to be banned in competitive leagues. The Australian Christmas update reduced the effect of the slowdown to only work at close range, and increased its spin-up time so that Natascha-wielding Heavies were easier to ambush.
The Equalizer was quite controversial among many circles before its nerf. As the player lost more and more health, he started moving faster and dealing more damage while it was the active weapon, meaning that it became quite possible to one- or two-shot three or more enemies in a period of a few seconds, as the damage let you kill almost any class from full health in two hits and the speed made you nearly impossible to hit. The flip side is that Medics can't heal you while it's the active weapon, which is easily remedied by taking out any other weapon, but when you're moving around so quickly and dealing so much damage, why would you want to be healed? As a melee weapon it was limited in how game-breaking it could be, but most players considered it a pure upgrade over the regular shovel, and Valve spent several patches nerfing it, finally culminating in splitting up its speed and damage attributes into two different weapons, with the speed portion going to the Escape Plan.
The Escape Plan was nerfed even further due to players still frequently using it as a scot-free escape mechanism whenever a fight went the wrong way. Now, having the Escape Plan deployed causes the Soldier to take Mini-Crit damage as long as he's holding it, and for a short while after you switch weapons, making it tougher to make daring getaways.
When the Spy vs. Sniper Update came out, the Spy got two new invisibility watches that could (1) let the Spy stay invisible indefinitely, or (2) fake his death and become essentially invulnerable for 6.5 seconds while ignoring the bump-into-reveal-cloak penalty. Not so bad by themselves since they also had significant drawbacks, but at the time of the update, the Spy also got his own taunt kill, which could still be performed while invisible. And killing the Spy mid-taunt could mean nothing. Valve spent several patches fixing these oversights.
The first release of the Dead Ringer let him cancel it immediately. Add the fast recharge and as long as you constantly clicked to keep it active and cancel any faked death, you had about 1200 HP (ten times the normal amount or so). Compare the highest-HP class, the Heavy, with his smashing 300 HP, the Spy was basically immortal until a patch fixed this. Now it drains down to 40% every time you cancel, as well as having a cooldown of sorts.
It was still broken when it started draining at cancel, because you could decloak (loudly) right before touching a big ammo box, and then immediately ready it up again. Combine this with the typical player mentality of "kill enemy > capture objective", and you could take an easy MVP on defense with virtually no kills by just being an unkillable pest near the attacking team's spawn. In particular, Gold Rush stage 2 point 2 was virtually impossible to lose because of the abundance of ammo back near point 1. This was eventually fixed by severely reducing the amount of cloak that can be gained from large ammo boxes while holding the watch in question.
Even despite all that, the Dead Ringer still demanded nerfs, since the feign death feature was abused by bad Spies for the damage resistance. In addition to promoting bad tactics (using it to tank damage rather than fool the enemy), it served mainly to set back the enemy while the Spy has plenty of time to run to a health and ammo pack, even while on fire. In the end, the Dead Ringer was nerfed so that taking enough damage while cloaked will revoke the damage resistance and no-flicker of its cloak, making sure that those useless Spies don't overstay their welcome (A Dead Ringer Spy will now die to a Pyro in about 3 seconds at most.)
The Sniper's unlockable primary, the Huntsman, also falls into this category for some. You would think that a bow-and-arrow that does less damage than the Sniper Rifle, carries less ammo, has no scope, and is less accurate would be inferior, right? Not if it uses the flamethrower's hitscan box to determine hit detection! Not only is the hitbox for scoring a successful hit on a target at least twice as big as the target when you're using the Huntsman, arrows that fly close enough to the head will score a headshot. Ever wonder why that Sniper that you ambushed from behind was able to suddenly turn around and headshot you at point-blank range? Now you know.
The Engineer Update set a new standard for turtling. One of the Engineer's new weapons was the Wrangler, which allows an Engineer to take personal control of where his Sentry Gun shoots. Not bad by itself, but consider the fact that this gives the Sentry infinite range (along with the fact that it has no damage falloff, meaning it does full damage at any range), and among the Wrangler's features is that it doubles the Sentry's fire rate (which is is basically double damage), and projects a shield that causes it to take only 33% damage, tripling its HP. With no downside apart from disabling the Sentry for three seconds when the Wrangler is un-equipped or when the Engineer is killed. Instantly banned in competitive play.
Among the new features in the same update is the ability for an Engineer to pack his buildings up and move them. Not too bad by itself since it's generally a wise idea to be able to move Dispensers and Teleporter Exits forward as your team advances. But when you can move your Sentry Gun as well, and combine it with the Wrangler... you can take over and lock down an entire area given about six seconds of nobody noticing you.
The Engineer can also use the Sentry's knockback to shoot himself into otherwise unreachable locations with the Wrangler (called Wrangler-jumping or Sentry-jumping) and set up nigh-impassible choke points, especially not when supported from traditional Engineer campsites by other Engineers. Additionally, some of these places could still be built on prior to some major map overhauls, so was possible to build a Teleporter Exit that cannot be reached by the enemy team.
Eventually, Valve decided to make long-range Wrangling less effective by reducing its accuracy at long range, while reducing the after-death shield duration from 3 seconds to 1. This made it easier for Snipers to simply pick off the Engineer, and then have plenty of time to destroy the Sentry since it wouldn't resist two-thirds of the damage dealt to it.
The first Polycount Update (along with Australian Christmas) brought this with the advent of 'set bonuses.' Having an entire set of equipment grants bonuses and flaws on top of existing ones. While bonuses and flaws aren't exactly new, what made it game-breaking is that some of these sets didn't come with any flaws at all, meaning that somebody with the item set hat had a pure advantage over someone who didn't (for instance, a player with the Special Delivery set while wearing the Milkman has 25 more HP than a player without the hat, while carrying the exact same weapons loadout). This was eventually rectified by making future sets purely cosmetic or having small enough bonuses that they didn't matter, and the old ones had their bonuses merged into the weapons.
The Scout's Mad Milk was this for quite some time. Anybody hit with milk became an instant HP replenishment sink for anybody who had the common sense to shoot at them, since any damage done to a milk-covered player would return 75% of that amount as health to the attacking player. This meant that if you had a high enough damage output, you could essentially heal yourself about as fast as a Medic can, without the Medic. Thankfully, this was later toned down to a more reasonable 60%.
The Soldier's Battalion's Backup was also this for a time, though for different reasons. You charged it up by taking damage from enemy attacks, and once you get to a certain point, you just let it loose and any teammates close by becomes immune to critical hits (including headshots) and got a 35% damage resistance buff. By itself, this wasn't so bad since the effect only lasted for 10 seconds or until the Soldier died (and smart players always target banner-wielding Soldiers as high-priority targets), but the Soldier only had to take 175 damage in order to fully charge it; given that the Soldier has 200 HP by himself, and even more given Medic support or by using the Black Box, it virtually guaranteed that a Medic-backed Soldier could have an active banner going around 50% of the time. In addition to this, fall damage still charged the banner, so it was possible to charge the banner during setup simply by rocket-jumping repeatedly just outside of spawn. The damage requirement was doubled to 350 damage later though, and various methods of self-charging were also removed, though map effect damage such as burning torches and drowning still charged the banner before its charge method was changed altogether to dealing damage rather than taking it.
Ironically, the damage absorption nerf ended up producing an inversion of this trope, namely that it was so high that players didn't want to take the risk of absorbing 350 damage without dying just for some minor buffs, so Valve changed the weapon to make it charge through dealing damage. They also buffed it so that the boost has an additional 15% Sentry resistance, causing Sentries to deal only 50% of their normal damage to players that have been boosted with this weapon.
In general, the first set of Polycount item sets drew a lot of hate and ire since they required players to wear a hat alongside the items, and hats are a hell of a lot harder to come by than weapons, both in random drop and in crafting. So, for the first few weeks of the update (and for months afterwards), the only ones who were able to take advantage of the item set bonuses were the ones that either got lucky in item drops or crate openings, were farsighted enough to save up enough Refined Metal to craft one of the hats before the update, or simply purchased the hat from the Mann Co. Store.
With the Australian Christmas, another batch of item sets came about. However, out of all the new weapons, the item sets were not as hated (they weren't as good, requiring a huge tradeoff for a minor bonus, on top of not requiring the hat for the set bonus this time around) compared to Heavy's Fists of Steel. Sure, you take double damage when hit by melee weapons (usually resulting in OHKO when engaged in melee battle, especially against a Pyro with the Axtinguisher), but the fact that it provided 60% damage resistance against ranged weaponry allowed Heavies to survive fully-chargedheadshots, as well as Sentry barrages. Thankfully, this was later reduced to 40% and a weapon switch time penalty was implemented as well.
At one point, it got bad enough that a Heavy equipped with the Fists of Steel, while backed by a Medic, could wade into Sentry Gun fire on Capture the Flag maps, grab the Intelligence, and simply waltz back out, as the combination of overheal and damage resistance meant that the Heavy had a total effective HP of over 1100, and was being healed for effectively 60 HP/second, just under the fastest rate at which a Medi Gun can normally heal players. The same thing could be done with carts on Payload maps, completely shutting out most push or defense attempts, with exceptions such as an Axtinguisher-wielding Pyro.
Things got hilarious when a glitch was discovered, where disconnecting from the server after playing the Amputator (which gives a small amount of healing to everyone in the Medic's radius) would cause the healing effect to stay. Combine it with the Fists of Steel, and it was possible for a Heavy to wade into point-blank minigun fire and punch the enemy Heavy to death without dying.
The ‹ber Update gave the Heavy the Tomislav, a minigun with a silent and fast spin-up that does slightly more damage than Natascha. In effect, the faster spin-up time turned the Heavy into the most powerful ambushing class in the game, even moreso than the Pyro, who was designed specifically for ambushing enemies, because the slow spin-up time and the loud whirring of the Minigun's barrel were supposed to be the Heavy's greatest weaknesses. Immediately after the update hit, Heavies could sneak up behind their enemies and mow them down effortlessly, to the point that the reduced damage output didn't matter at all. Another problem was that since it spun up faster, the slower firing speed didn't matter at all since the time saved spinning up could be used to kill most weaker enemies faster (which makes a large difference in the case of a Medic) versus the regular Minigun. So trying to sneak up on a Tomislave Heavy to perform an ambush was futile, since they could turn around and mow you down half a second after you land your first hit. A patch which lowered its spin-up time reduction from 75% to 40% somewhat rectified this, and an additional patch almost completely eliminated the spin-up bonus.
The Enforcer was another ‹ber Update weapon that was highly controversial upon release. For a 20% damage increase, its only disadvantage was taking a little extra time to become invisible... something easily negated by using the Dead Ringer, which instantly cloaks regardless. The additional damage allowed Spies to fight on the front lines and deal a surprising amount of damage to pursuers, which completely eliminates the Spy's class disadvantage of not being able to fight back against other classes in a direct fight or when discovered. This got especially terrible when a certain glitch with the Diamondback allowed players to exploit infinite crits with the Spy's primary weapon. However, it has since been nerfed and now has a slower firing speed, only causes the damage bonus when the Spy is undisguised, and can't deal random critical hits.
Upon initial release, the Cow Mangler 5000 was quite unbalanced. Besides dealing reduced damage against buildings and its inability to be crit-boosted, it had no drawbacks compared to the stock Rocket Launcher; however, it has unlimited reserve ammo AND 5 shots per magazine instead of 4, allowing Soldiers to do huge amounts of damage while away from support, and its damage ramp-up was unintentionally too high, meaning that it did more damage than the Rocket Launcher at close range. On top of all this, it has a special charge attack that lands a guaranteed Mini-Crit (which also sometimes did more damage than normal on top of the increased ramp-up), ignites players hit by it, and disables Engineer buildings for 4 seconds; this allowed Soldiers to disable Sentry Gun nests from a distance without the need to get a Spy close-in, and possibly kill the Engineer tanking it. Then there was the fact that it was a literal game breaker in that its particle effects taxed some graphics cards beyond their limits. Virtually all of this was fixed shortly after the update that introduced it, although later some of its penalties and bonuses (the extra shot per magazine and reload/damage penalties) were removed.
The Machina is this for some. It does an additional 15% damage on a full charge, making it capable of body-shotting anything short of a Soldier or Heavy at full health on a full charge. It has two downsides in exchange: it fires tracers to tell the enemy exactly where you are, and can't shoot while unscoped. If you're camping in an entrenched position where the enemy can't reach you and since death-cam will reveal your position anyway, then neither of these disadvantages matter and the weapon is a pure upgrade over the regular Sniper Rifle. Because of this, the Machina has a reputation among the community as a weapon for "inferior" sniper players who can't/won't headshot.
The Gloves of Running Urgently, originally released in the Polycount Update, gave the Heavy unprecedented mobility for virtually no penalty. While it was the active weapon, the Heavy lost 6 HP per second, but had his movement speed boosted to match that of other classes, allowing him to keep up with teammates and move to the front lines much more quickly. The health penalty was originally intended to penalize the Heavy and force him to work with teammates, especially Medics, in order to maximize its usefulness, but when combined with the Sandvich's ability to instantly heal, this penalty suddenly became useless, and Heavies could additionally simply equip the GRU and make a fast getaway for no penalties if a fight turned the wrong way. The health drain penalty was eventually replaced with taking Mini-Crit damage while the weapon was active and for 3 seconds afterwards, forcing Heavies to utilize them more strategically.
The Heavy's Sandvich was somewhat overpowered for quite some time. One of its functionalities that was added some time after initial release was to use alternate fire that allowed the user to throw it down and have it act as a medium health pack. Not broken on its own since it encouraged players to share their food with their teammates, but this also applied to the Heavy who threw it, allowing him to heal 150 health nearly instantly, which made it virtually impossible to kill a fleeing Heavy (or one wounded inbetween engagements) since they could throw down their Sandvich and instantly replenish health on the move. Combine this with the Gloves of Running Urgently, and you suddenly have a very mobile Heavy that can get to the front lines for negligible penalties. The devs later removed the self-healing feature, only allowing a Heavy's teammates to pick up the Sandvich for health, while the Heavy must physically eat it in order to regain health.
The Pomson 6000 got a lot of hate for its low risk vs. reward and capability to easily counter two of the Engineer's counters upon release. Its normal attributes weren't anything to write home about, and its ability to penetrate targets was only really useful when holding down a corridor due to its mediocre damage at long range. The main problem came from its ability to strip 10% ‹berCharge from a Medic for each shot landed; just hitting a Medic twice (which is really easy to do given the projectile's large hitbox detection) was enough to set their charge time back by 16 seconds, long enough to create a window with which to kill the Medic while he's helpless; this becomes a major problem when a Medic has a full ‹berCharge meter, charges forward, only to have it taken away the second before he's able to pop it, completely shutting down a push attempt. Teammates trying to body-block these shots, the natural tendency when trying to protect a Medic, were also wasting their time, given the Pomson's ability to penetrate targets. The Mann vs. Machine update removed its penetration ability, greatly reducing its spammability.
The same weapon also made Spies ineffective. While it didn't affect Cloak and Dagger Spies (although they could be found much more easily simply by shooting everything with a pistol) it heavily affected the Dead Ringer Spy. The Dead Ringer can only activate upon a full cloak, so being glanced by one shot meant you had to wait several seconds before you could actually cloak. A sharpshooter Engineer could hit potential Spies long before they realized what had happened and deny them fake deaths in enemy territory. It also didn't help that the Pomson made a tell-tale noise whenever it drained Cloak or ‹berCharge, whereas every other hit-related sound in the game is disabled if you hit a disguised Spy. Suddenly Pyros were no longer needed...
The Spy-cicle, similar to the Enforcer upon release, is highly controversial for what it does. While the silent killing feature is offset by turning its victims into easily spotted ice statues, the real issue is that if the Spy is set on fire, the Spy-cicle will instantly put the Spy out and make him fireproof for 2 seconds, in exchange for not being able to use it for 15 seconds. This nullifies (or at least drastically reduces) the threat posed to Spies by Pyros, which is supposed to be the Spy's greatest class weakness. When combined with the Dead Ringer, you have a virtually invincible Spy that's almost impossible to Spy-check. Throw in the Enforcer and this was one of the most infamous Spy loadouts before the Enforcer's nerf.
The Pyro's Phlogistinator was a fairly controversial weapon. In exchange for having no airblast, Pyros gain a "Mmmph" meter that tracks how much damage the weapon does. Once it's built up enough, they may activate a taunt that gives them guaranteed critical hits for 10 seconds, while restoring them to full HP. Normally, this wouldn't be too bad, but for the duration of the taunt, the Pyro gained 90% damage resistance, essentially making them invulnerable. This allowed players to stop in the middle of the battlefield and taunt without any repercussions until after the taunt finished. Additionally, the required fire damage in order to activate the taunt is a mere 225 points, which is easily achievable (just kill one or two enemies and ignite a couple others), and the critical hit boost is longer than the Medic's Kritzkrieg ‹berCharge. On top of that, fire damage wasn't limited to igniting enemies with the Phlogistinator, using a flare weapon could guarantee that you would have a full meter before even getting within flamethrower range of an enemy. In addition, "Mmmph" gained from fire damage is not affected by fire resistance, so a dead ringer spy or a targe demo (see above) could completely fill the meter despite not having enough health to do so normally. Thankfully, Valve toned this down over various patches, culminating in reducing its damage reduction to 75%.
The Red-Tape Recorder is one of the most effective ways to make Engineers switch classes, rage quit, or throw their computers out the window. While it is incapable of destroying buildings unless the Engineer leaves it alone for upwards of 15 seconds, and deals no damage to buildings at all (meaning that it doesn't help teammates in destroying the building, outside of paralyzing a Sentry), it is capable of de-leveling buildings in a matter of two to three seconds. Not only does this force Engineers to upgrade their buildings all over again, but also virtually guarantees that a building will lose a level, if not revert entirely back to level 1, even if the Engineer was standing right next to the building and started hitting it the second the sapper comes down. Also, because they de-level buildings so fast, it highly discourages Engies from fighting back against the Spy, so the sapper can be continually spammed. Even against Engineers using the Gunslinger, the Red-Tape Recorder still paralyzes the Mini-Sentry for up to 10 seconds, meaning that the Engineer is incapable of building another one to defend himself for that long, making him easier to kill. The only way to effectively counter is to have a Pyro holding the Homewrecker watching over a Sentry, praying to react fast enough to prevent a decrease in level. This was alleviated somewhat when the downgrade time was extended from 1.8 seconds to 3 seconds. It didn't help that a glitch could give a sentry loads of health at one point
The Pretty Boy's Pocket Pistol gets a fair amount of hate for stepping into "pure upgrade" territory. Simply equipping the Pocket Pistol gives a Scout +15 max health and prevents him from taking fall damage, greatly increasing his survivability against most other classes. In exchange, it makes him considerably more vulnerable to fire damage, and has a slower rate of fire than the stock pistol. The first disadvantage only matters against Pyros, which Scouts are strong against anyways, as any competent Scout can kill a Pyro at any range without even being hit. The second disadvantage only comes into play when Scouts run out of ammo for their primary weapon and need to finish an enemy off, at which point their target is already at low health and virtually guaranteed to die anyways.
And so on and so forth. Every time an update ships, you can be sure that somebody will be complaining that Metagame has been completely ruined.