I love #tf2mix. I, too, was not around for the "good days", and certainly I'm guilty of some of the idgaf QFing (although I do it rarely (I prefer semi-serious QFing)).
Some of it's the captain system, and troll momentum. People seem reluctant to leave mix when they have a team they like and go somewhere like tf2lobby to dick around, so they surf the captain games and guarantee they can get the same team consistently and stomp/troll. I've medic captained at times specifically to stop this, because I hate fun.
There's also been some ill-advised distribution of mumble admin in the last 2 days, which I'd reconsider. I hate seeing my pocket jerked out of mumble by some kid who thinks it's funny to dick with his friend without regard to the rest of the team that's actually playing a semi-serious game.
But... I mean, I also don't think I appreciate where the resistance to meta changes comes from. The 6s class distribution is what it is because of an optimization process that happened. If a team's running pyro to mid, show them why a scout/soldier is better. Kill them. Similarly, a team shouldn't be able to get away with spawncamping forever. Figure out a way out, even if you have to wait for respawns.
Also, it seems like, in mix, there's no punishment for leaving. If someone's seriously wasting your time, or you can't handle your team, request a sub and don't stick around.