I've noticed while playing online there seems to be a slight hate of the Pyro. Many people treat it like you can be a good Pyro by holding the left mouse button and moving forwards.
So here's a guide thing on why the Pyro is a good tactical class and not just a newbie's fireball party.
The Flamethrower- The Pyro weapon of choice, depending on whether or not you've unlocked the extra weapon yet. Any situation can most likely be fixed with this, if not, it's not a situation for a Pyro and you should get out of there.
the Flare- I admit, I haven't unlocked the flaregun yet. But it appears to be very useful, since, unlike all other Pyro weapons, it can attack at a range. Ranged attacks should not be your focus, but if you have to, make sure you have one handy.
There are two Pyro tactics that I use. The runner Pyro and a sneaky Pyro. A runner will most likely charge screaming his head off into enemy lines, burning everything he can before he dies in some desperate kamikaze. It sounds quite silly, but sometimes dying in that way can screw the enemy over and ensure a victory. A sneaky pyro is the kind that sticks round corners and jumps out of unexpected places, a good sneaky pyro is like a spy, except on fire.
Tips for being a good spy detergent-
-Spies always use a primary weapon, Medics and Engineers almost never use the primary weapon. Keep an eye out for these guys.
-Pay attention to the team, if someone who wasn't there before, suddenly is there, it's probably a good idea to check them.
-If you see an undisguised spy running around, it might be an enemy spy. Check them.
Random tips for battle-
-If defending, hide round a corner and wait for the enemy to attack. Especially in tight corridors. It can give the momentary panic you need to pump them full of fire, and at the same time, the Pyro can keep up with most classes (but not outrun them) so even after the panic cools down, burning still commences.
-Inside is your home. Command points are all inside. A quick Pyro strike can make the difference between a control point and an enemy blockade.
-Spies are afraid of Pyros, but at the same time, they want to get rid of you. Stay on the offense and they should be forced to leave you alone. If on defense, keep an eye out and always check anyone who is acting suspicious.
-If going for a sneaky pyro, make sure to be patient. Whereas most Pyros will never stay in one spot for an extended amount of time, it's sometimes worth waiting to blast a heavy/medic combo before they get that ubercharge working.
-Never join in combat with a Sniper. If you are even killed by a sniper, there's something wrong there. You're playing two different games here, he's outside, you're inside.
-You are not a defense class. Keep on the offense unless you HAVE to defend.
-Never focus on an individual if fighting a group. Burn everything, if there's a big threat, like a heavy/medic then of course you can burn away, but burn the rest as well first. It'll weaken them for when the heavy/medic are dead or when the rest of your team arrive.
-You are slow, if you were fast, you'd be unstoppable. But for now, you are slow. Don't try to fight a faster class, they will dominate you.
Tips for counter-acting a Pyro-
-Be fast. Pyros are slow and rely on killing enemies before they die instead of agility.
-If a Pyro is out in the open, keep your distance and shoot him from afar, otherwise get outside and he shouldn't follow you, if he does, do the same as above.
-If playing as a spy, a Pyro is a nice target. They are very powerful against spies, but many Pyros will understand the threat they pose to spies and will expect them to keep their distance.
-Be cautious when moving through tight corridors, Pyros love it in there.
-The flamethrower makes a distinctive, yet quiet noise when used. If you hear it, expect a flame attack.
-Be wary. Pyros are not designed to win a battle, they are designed to let the team win the battle. If a Pyro attacks, remember that he might not be trying to kill you, just to weaken you. Keep a medic near.
-If playing as another Pyro, you will have a problem. Rely on hit and run tactics, or try to lure him out into the open for your teammates to take him out.
That's all I can think of off the top of my head. Results may vary.