To respond to the points made in the first post:
Snipers, yes, they can be very inefficient. Especially on the map "hightower", on that map snipers have two great vantage points, overlooking a large part of the map and mostly, the enemy snipers vantage point! The result is that snipers on that map just keep each other busy! And because it's such a great map to play sniper, you see that sometimes half your team plays sniper!
I don't agree with your anger about pyros who don't airblast teammates. It costs a lot of ammo! I only use it when a teammate is close to dying or when he is a friend of mine.
Things that annoy me most:
Medic's that blow your cover as a spy by healing you.
Engineers on MvsM servers who build their sentry near the hatch in the beginning of the game.
Engineers who don't build teleporters (Seriously, why won't you build that? Do you realize how easy it is to score points when you're the only one with a teleporter?!)
Engineers who distract the sentrybuster, only to make it explode in your fucking face!!!
That object scouts can drink that makes that you can't hit them (what is it called?)