Shut-Off-Your-Mic!!!!!
That's one word, right? Some of the worst lag I've ever seen. (My legionnaire, keeper, archer, alchemist against threatening demons) All three of us off-hosters were jumping all over the place, with no clue where we were. At one point it took me 5 tries to climb a staircase! And just to make it better, someone had their mic on, and seemed to be playing in the living room. I could hear the TV, a dog, a bird, his sister talking on the phone, and his parents(?) talking about the movie on the TV. Once his sister ended the phone call, he spent the rest of the match having a conversation with her...
And yet, somehow, we made it to the boss fight, and actually cleared out all the mobs until it was just me and the boss left. Unfortunately, trying to solo the demon commander with a melee character with that kind of lag just wasn't working.