Scout: Use your attributes to your advantage; you're fast, you have some useful items, and you have a damned fine special ability.  Speed is the key when you're on offense.  Try not to use grenades unless it's necessary to get someone out of your way or to keep them from tailing you.  It's especially important on offense to be careful with caltrops and concussion grenades.  You can still hurt your teammates with those items, and it can seem like they "get in the way" of them pretty easily.  Just remember two things: never drop caltrops when a teammate is directly behind you, and always look left, right, then left again before throwing a concussion grenade across the street.  The crowning achievement of the scout, in my opinion, is his ability to defuse detpaks set by demolition men.  Whenever you see a detpak (unless it's set by your team), try to defuse it!  You'll be regarded as a hero by all of your friends for saving their lives, and if you die with it, you can say you tried(note that this does not apply when you are a lone offenseman and the detpak is about to go off... no point in committing suicide then).  Note: if possible, the scout should be the only class to carry a/the flag in most maps(except maybe canalzon and tater)  

Sniper: Many people call him the backbone of any team's defense (including the United States Army, Navy SEALS, and the Marines), and he is just that.  Your gun is a powerful weapon, and as such rips through armor like Oprah through a roasted turkey; use it with caution, you don't want friends coming back with a peg-leg to beat you with.  Snipers shouldn't go off on missions of offense unless a)your defense is holding up just find without them (in which case you should probably go an offensive class), b)there is a sentry that nobody else can get, or c)you're more l33t than a PII 700 w/ 256MB RAM with a T4 on its own backbone (in which case you could single-handedly destroy the enemy defense and offense with your axe).  Your grenades are pretty simple; hand grenades are a no-brainer, just don't toss them to a friend, and flares are meant to be put in the enemy fort's shadow sniping areas, NOT as party favors for your teammates.  

Soldier: Once again a rocket-toting maniac has shown us the way; same ammo deal, same grenade deal (just for fun, try tossing a nail grenade timed correctly into the sky over the bridge; if you do it righty, you'll have a hail of nails for your enemy snipers (make sure your snipers are tucked away under some heavy bricks)).  Just remember, you can take a lot of hits, your faster friends can't.  Can you say human shield?  

Demolitions Man: This guy eats shrapnel for breakfast, but it doesn't mean his teammates have to; keep your grenades out of the base unless you are pipebombing enemy-trafficed areas (flag and ramps in 2fortX).  Keep detpaks out of your base, too, nobody likes to bounce off a ceiling and lose all of their armor.  

Medic: The most helpful support class in TF to players.  Use your medkit often (and if you're not a medic, stay outta his health boxes, he needs them!  It's far more useful when he can heal you completely with 1 whack and a small amount of health stuff instead of you taking 4 boxes), and use discard frequently.  If you are on defense, run to the supply areas frequently, pick up some rockets and shells, and discard/dropammo them to your friends.  They will never have to resupply (if you also have an engineer on hand to fix their armor).  Like the scout, you will have to watch your concussion grenades; if you chance to hit a friend with one, take the time to heal him (however difficult it may be in 2.8).  

Heavy Weapons Guy: Like a demoman, you must watch your spam, but you must also stick to defense.  That chaingun is great for covering people running out from your base, too.  

Pyromaniac: Another support class, he is good for primary defense.  First you toast your enemies, then your friends can finish them off with the barbecue sauce.  Try not to go too far on offense unless you are a bullet cushion and soften up enemies, too (note: this class is good on offense for getting rid of sentries because of his napalm grenades).  Also like the demoman, try to keep your spam out of the base at all costs.  

Spy: Perhaps the most useful class to your team's offense, you should take out sentry guns, snipers, toss a gass grenade into a respawn area, and tell your teammates about locations of enemies and other goodies.  Try not to change color in your own base, it just confuses friends.  Also, notify your teammates of who you are disguised as, and if they shoot at you, just jump like a mofo and have a key bound to say_team "stop shooting the jumping mofo!".  Oh yeah, those gas grenades are a pain, try to keep them about half a mile away from your base.  

Engineer: An MIT graduate, he is good for defensive support.  Build sentries and dispensers, heal teammates' armor, and watch where you throw EMP grenades (you don't want to blow up all your team's ammo)  Also, building sentries in the enemy base is often considered cheesy.  Use your head and only do such a thing in the right game and map.  

Thank you for reading this guide, it has been quite a pleasure to write and HTMLize!  E-mail me with any suggestions/additions you may have, and remember that I am planning on updating it as often as possible.  Before you ask, yes I do plan on making one of these for TF2 =)