IRC

That's the biggest reason I wouldn't go back to IRC. I mainly participate in channels where the volume is low enough that I can read every message, and I don't need to worry about missing potentially important or interesting conversations that happen while I'm asleep or on holiday etc. And, crucially, I know the other participants won't miss conversations either - I can reply to something they said hours ago without having to care whether they're currently online or not, and without having to switch to a different asynchronous messaging platform (like email).

I used to use IRC with irssi+screen+ssh for persistence, and it sounds like Quassel provides similar functionality with a slightly less rubbish UI, but it still doesn't allow that semi-asynchronous communication unless everyone else is using the same non-default client (and they almost certainly aren't).