I can wholeheartedly say that there is only one thing that truly pisses me off more than anything else in this game.
Nevermind the Huntsman arrows that come flying across the map and should rightfully miss you, but since they have the bounding box of a bus, they murder you anyways.
Nevermind the fact that Heavies always manage to pop up somewhere and you're right at point blank range and they kill you in under a second flat.
Nevermind that the Demoman will miss every grenade he shoots at you except the critical shot.
Nevermind the Soldiers who do the same thing with rockets.
Nevermind the Scouts who are painfully accurate with their scatterguns and can drop you before you even hear them coming.
Nevermind the Righteous Bison projectile looking almost exactly like the Cow Mangler projectile, so in the heat of combat you can't tell which is which and waste time, ammo, and health trying to airblast it.
Nevermind Demoknights who seem to have a ten yard melee radius with all their weapons, not just the Eyelander - the only weapon that explicitly says it has an extended range.
I am sick and fucking tired of the slightest bit of lag throwing everything off for me. Try to land a Fan o' War hit to mark someone? They lagaport behind me, and kill me. Try to close in to an effective range for my Soda Popper minicrits to be dangerous? They lagaport 90? to the side, and kill me. Try to close in for any meaningful use of a flamethrower of any flavor, and the server assumes I'm lagging so the flame projectiles are behind me (I literally had this happen. I was running forward, spotted a Pyro just outside of range, and instinctively sprayed a puff of fire... and ignited the Spy that was right behind me who in turn backstabbed me). Try to flank someone with the flamethrower, and again, the flames are three feet behind me, meaning I need to be in front of them, spraying at an angle so I can't see where they are anymore... and they kill me. Try to maneuver around someone as a Scout, using my superior agility to my advantage, but somehow I'm apparently trying to run through my target, and we're both wearing suits made of velcro... and they kill me. Try to airblast grenades and rockets as they get shot towards me at medium-to-close range, but somehow the Soldier does the firing animation, the rocket launcher shows the puff of smoke and the recoil, but the rocket doesn't go anywhere for a second, just hovering in space before hurtling towards me... right after my airblast, leaving me on cooldown, so it kills me.
If anyone is playing a Sniper, they should have top priority for server connectivity, followed right after by Pyros. Those two classes really need to be completely up-to-date on the true location of other players. I'm tired of the server thinking I'm fixing to sprint forward on the battlements of 2fort, when I'm just taking a step, and remain completely behind cover... but get sniped anyways. From an angle that has absolutely no line of sight to where I'm standing. Because I projected an image of myself poking out from behind cover that lingered for just a moment, but since 2fort is a Sniper breeding ground, there's no less than two Snipers staring at where I 'am'. I'm tired of being sniped after I've turned two corners and am a step and a half away from a health kit. I'm tired of trying to airblast rockets just for them to sit still for a moment before deciding they're ready to start flying. I'm tired of trying to close into melee range as ANY class - Demo, Scout, Pyro, Sniper - and having my target start immediately lagaporting around me, flailing their weapon around like a moron, and suddenly my head flies off on a jet of blood. I'm sick of actually facing the Spy who was just creeping up behind me, and seeing him swing his dagger, but it still registers as a backstab because he's playing with 380 ping, so my reflexive whirl upon hearing his Dead Ringer turn off didn't show up on his screen yet, and the server takes pity on him.
Everything else annoys me slightly, but I can handle it. I can even handle exceptionally talented Spies who are good at what they do and manage to dominate me with backstabs, keeping me dead for half the match.
I can't stand a kill being snatched away from me, or handed to someone, just because they're playing on dial-up.
Also,
Also if you shout medic, please don't equip the equalizer. I cannot tell how many times I've seen this happen, and it kinda gets on my nerves.
I tell the Soldier in teamchat that I won't heal him if he does it again - first because I can't while it's out. After that, I won't heal him period, so he needs to make up his mind. It's especially annoying having them whip it out while you're Ubering with the Quick Fix. Just cuz you don't turn all shiney doesn't mean you're not being Ubered.