Opt-out chat for combat experiences is exactly what scares me, honestly. Because as weird and off-beat as chat can be in social spaces, giving people who have no contextual relationship like friends or a fireteam a comm channel is an avenue for a lot of abuse.
You might be willing to patiently explain the Taken Blight for new plays (at least for now, whether you'll do it for people who buy the game three months from now remains to be seen) but a lot of people will just rip and chew into players for "doing things wrong". The mentality is: "They're effectively strangers who I won't see again, so what's wrong with a little (well deserved) abuse to get them into line?"