Having destructive super powers like AOEs and novas and ONLY hitting the bad guys even though your teammates are standing right next to you
This is pure game play, "Friendlyfire" in MMOs is outright the worst singular idea you could do, it would mean that you could effectively never use an AOE attack for fear of a blaster accidentally edging into it, Defenders could -never- use their toggle debuffs, nor could dark..no patch effects could be remotely effectively used.
Oh and MMs would be unplayable.
This has been tried, Dragon Age has the option for it in at least the first game..and I have never, ever seen anyone use it.