1. Enemy pets/summons that give no xp but don't go away when the summoner does. If the summoner's gone, their minions should go as well or if they persist and stretch a fight out, gimme something.
2. Certain enemy attacks/holds will detoggle something and it's oh so easy to not notice and get smacked by something with no defense against it. I've had characters get hit with something and only later did I realize one of their defense toggles turned off.
3. Every. ##$%ing Freakshow rezzing in a mission/TF. Here and there, sure. When it's all the time (and it does happen) things draaaaaaaag. It's already been proven you're not winning, stay down.
4. Not being villainous enough Redside. There's a handful of story arcs/missions that will make you raise and eyebrow in how twisted they can be, but most Redside content is being an anti-hero at best and total loser at worst.
5. Everyone Goldside has super reflexes and an invisible targeting drone. Yeah, I know it's meant to be more challenging, but sometimes it dips into absurdity.