The usual:
Pyros - Specifically the retarded physics of the airblast that rob you of any momentum you had before the blast, and make any significant aircontrol impossible. Also, the retarded range of the airblast. Also, the way airblast works when it shouldn't (I'm soldier, pyro in front of me, I flick back/down for a rocket jump, pyro spams airblast button, gets a deflect kill even though the rocket for my rj was already going the complete opposite way of the pyro... )
Minisentries - One thing that seriously has to go is the full-hp on deployment. A deploying minisentry should die with 1 rocket or 1 pipe, just like a regular sentry. I believe it used to be like that but Valve actually decided, nah, lets buff the minisentry some more??? Add to that the fact that metal-stacked engie can always directly deploy another minisentry (that again requires at least 2 shots) makes it even more annoying. At least one of these 2 unnecessary buffs has to go, and preferrably both.
ALSO: If an engineer can dominate you purely by his sentry killing you 4 times, then why does killing the sentry in return not grant me a revenge? Why does destroying his sentry 4 times not dominate him? Should go both ways!
And yes also the usual pub frustrations, like the medics who heal only 1 guy even though he is fully buffed and not being shot at, medics who think they should only uber if they are at 5 hp regardless of what else happens, idiot disguised spies who can't be more obvious but run in your face at the worst times when you're low so you die by self damage cause they are too close in your face (or self damage + 1 knife hit), snipers and scouts that occupy teles, killing 10 guys and then being market gardened from behind by some soldier who wasn't doing anything else for the past 2 minutes and then taunts cause he's so proud he got an MG kill on somebody who was low and had no ammo... I could go on for hours :D