What I find most annoying are demomen with splendid screens in corridors, if you don't see them then they kill you instantly (usually) but if you do see them they can't do much. As a pyro I've made it a habit to airblast and shotgun those demomen.
As a soldier it's much harder if there's a roof over them but if they get blasted up then their charge stops and they're much easier to deal with.
That's what you get for trying to be a close range specialist.
Oh look, top of the page. I better find something...
Look at that tiny heart.