For myself, as someone who mains Medic, I mostly find myself being annoyed by people who spam E constantly with no situational awareness. If I'm actively ubering a heavy through a chokepoint, I'm not going to overheal the Demo. If I see three afterburned allies at low health, I'm not especially interested in covering the small amount of missing health from our heavy.
Honestly, I don't mind someone calling if they are out of my FoV. But otherwise, what kind of medic doesn't keep aware of the health of their front line. Or at the very least, have auto-pings enabled for low health allies.
Otherwise, I am generally annoyed by bad Snipers. I understand that players have to learn, but there's bots and sniper training for that. As, in my experience, if someone is going to be a bad... anything. It's always going to contribute more than a sniper that can't headshot reliably.