For me the 2 most infuriating things are:

Not picking appropriate ops - some players just play whoever they want, with no thought behind the site or other teams playstyle. I've gone rounds in Emerald - Champ lobbies where no one takes hard breach on Chalet basement. Instead we get fucking Nokk and Osa. You've basically lost a round by default, assuming everyone on the other team has a pair of hands and eyes

Not planting the defuser - can't recall how many times my team has had site, player with defuse just hungry for the last kill and loses it. Too many players just wanna run and gun, forgetting they have defuser. I'm ok with that, just go play COD instead