I had very bad luck yesterday - I managed to join the losing team every single game for 2 hours straight....
Demomen are a problem - they are waaayy too powerful en masse.
Another annoyance is the fact that most of the time when I join a team, there will be no medics, so I have to end up playing as medic a lot.
Also, new and inexperienced players choosing the spy and not using the class properly during defense is really annoying (why couldn't you be a medic instead??)









