For me, the worst is when the other player doesn’t go when he enters your lobby. I’ll kick any player who takes longer than 0.75 seconds to go when it is possible to go.
Then, you get the guy who sits at the character select screen for the full 40 seconds. I’ve taken to unplugging my ethernet cable if they don’t make a choice in 5 seconds.
What are the little things that irritate you online?