I've always been using #tf2mix for the past month or so. I certainly agree that some mixes are ruined by offclassing people that often don't do much, and end up dying after getting rushes. A lot of the time it's quite frustrating cause they can't get a pick anyways, therefore allowing the other team to push on your *useless* offclass.
I wasn't here for the establishment of #tf2mix, but I know that in #tf2.mix.nahl, the whole point of its creation was to help people get better. I'm assuming that in #tf2mix, the point of it is to create an area where not as seasoned people can play, but I have to say that when you have a team member that is oblivious it is annoying.
To address your point of people QF'ing: it's ridiculously irritating - especially when the medic does it for the sole purpose of fun.
Also, some of the time, people seem to just sit in IRC added up, go do something else for an hour, and then come back realizing they got picked. People can accept that, but it gets on nerves after multiple times.
Apologies for the rant but most of this is just the irritating stuff.