Team Fortress 2 Unlock Guide
Team Fortress 2 has been updated many times by Valves, and part of those updates has been the addition of new weapons and achievements for each class. However, these unlocks are not very well documented - which is why I've complied this list of all the unlockables and how to get them.
One of the best things about Valve, besides the great games they make, is that they support their games well beyond release. Team Fortress 2, which was released two years ago, would by now have been basically abandoned by most other developers. But Valve has continued to squash bugs with regular patches. They've also kept TF2 fresh by up by unleashing balance updates and new unloackable weapons which enhance and change how some classes play.
Currently, there are unlockable weapons and items available for the Medic, Scout, Pyro, and Heavy classes. These unlockables are earned through achievements associated with each class. When you complete a certain number of achievements, you unlock an item.
Once you have earned 10 of all the achievements for the Medic, you'll unlock the Blutsaugher. This is a new syringe gun which does not score critical hits, but it transfers a bit of the health from each hit against an enemy to the medic. It is not an exceptional weapon, which makes sense, at it would be bad to give the Medic too much offensive power. It does give the Medic a little bit of extra health, however, making it much easier to beat a fighting retreat. There is little reason not to use this once you earn it.
Once you have 16 achievements, you'll earn the Kritzcrieg. This is replaces the normal medigun with one that, rather than providing invulnerability as the ubercharge special, provides a 100% chance to perform a critical hit. This can obvious be devastating to a front line, although it is probably not as useful against defense positions that are defended by well-placed sentries. They are too deadly, and will kill whoever is ubered well before they can do much damage, unless you uber a Demo-man who is extremely good at what he does.
Earned when 20 achievements are unlocked, the Ubersaw is a replacement melee weapon which, unlike the regular saw, charges your ubercharge! This is less useful than you might think, because come on, how often is a Medic in melee range? Usually, it is only a moment before the Medic is stabbed in the face by a spy. That said, if you do manage to use it, then it is extremely handy. And besides, there's no real reason to carry the normal saw after you have this.
The Flare Gun
Unlocked when 10 achievements are completed, the Flare Gun replaces the Shotgun and acts as a long-range utility weapon. It does little direct damage, but sets the target on fire, doing a fair amount of damage over time. The damage done is not enough to kill anyone outright, but unless a medic is readily available, it will probably send them fleeing. The downside is since it replaces the Shotgun, you're left without a good mid-range weapon.
Earned at the 15 achievement mark, the Backburner is a flamethrower which looses compression blast and gains a 100% chance to critcal when hitting a target from behind. It also grants the Pyro an extra 50 health points. It is an extremely useful weapon which makes the Pyro a close-range ambush specialist, and even if you're not into surprising people, it is still often useful. Losing compression blast sucks when trying to separate a Medic from a heavy, though.
Earned when 22 achievements are unlocked, the Axtinguisher performs a critical hit against an enemy that is on fire. In practice, this is often a one-hit kill. It is weaker against enemies who are not on fire. This is a very useful weapon when you have an immobile or low-threat enemy like an Engineer or Medic backed into a corner, or when you ambush an enemy.
Unlocked at the 10 achievement mark, the Sandvich is not a weapon at all. It's a tasty snack! When eaten, the Sandvich replinshes 120 points of health for the Heavy over four seconds. The Heavy can't do anything during that time, so it can only be used in safe areas, unless you're suicidal. It basically gives the Heavy the ability to recover his substantial health pool which a Medic isn't in arm's reach. Taking the Sandvich means leaving your Shotgun at home, so be careful not to let your Minigun run out of ammo!
Unlocked when 16 of the achievements are complete, the Heavy's second upgrade is a another lovably named Minigun. The only difference between Natascha and Sasha is that the new mini-gun has a 100% chance to slow the target and does 25% less damage. Natascha is not as good in direct assualts against defensive targets, like a room with a sentry, other Heavies, Soldiers, and the like. However, when facing fast enemies like Scouts and Pyros, who like to try and get in close, Natascha is invaulable. You can also use Natascha as a spy check, since only enemies are slowed when hit.
Killing Gloves Of Boxing (K.G.B)
The Killing Gloves Of Boxing, earned with 22 achievements, are melee weapons which give a special bonus if you kill someone with them - a 100% chance to critical for five seconds. The uses for this are few in most situations, but it is a nice bonus to punch out an enemy spy and then go on a 5-second killing spree.
The first unlockable, the Sandman is a modification of the baseball bat. The first swing of The Sandman fires a baseball at the enemy unless an enemy is already in melee range. The baseball, if it hits an enemy, stuns an enemy for a short time, causing them to take less damage from your attacks but also rendering them completely unable to take action. This is useful against ubercharged Heavies trying to bust into your fortifications. The big downside of the Sandman is that the Scout can no longer double-jump.
Unlocked at 16 achievements, the Force-A-Nature is a special shotgun which knocks opponents backwards. This is considerably useful, as it can really disorient an enemy. It also gives the Scout the ability to separate enemies from critical points; for example, shooting a Engineer might knock him back from the Sentry he is trying to repair. The power of the recoil will propel the scout in the opposite direction when airborn, giving an effective "third jump". On the downside, the Force-Of-Nature can only fire two shells before having to reload.
Bonk! Energy Drink
This tasty beverage, available when 22 achievements are unlocked, replaces the pistol. Drinking it gives the scount temporary invincibility, although he can still be effected by knock-backs. When under the effects of Bonk! the scout can't take intelligence or attack. Bonk! has a cooldown associated with it, and after using Bonk! the scout is temporarily slowed to a speed only slightly faster than that of a Heavy! The best use of Bonk! is as a means to penetrate enemy lines. It can also be used to run out of a situation where the Scout is cornered, provided that you can find cover for the short time-period required to drink the Bonk!