One of the features of Gmail that I’ve long relied upon is the:
“Default: Reply to all, instead of Reply”
In 95% of cases, this is correct – it’s only rarely that you want to actively remove everyone else from the conversation.
…but this stopped working shortly after Google+ came out. Coincidence? I’m not sure.
Certainly, it’s caused MASSES of problems – people being dropped from email conversations, with associated obvious problems e.g. not hearing that a meeting’s been re-arranged.
I’ve bug-reported it to Google, but going on previous experience, there will be just silence.