Stop glomming on more and more recipients and CC-recipients in email discussions. Just don’t do it. You think you’re doing people a favor, “getting everyone involved in the discussion,” by throwing every conceivably remotely interested party into the To: or CC: line. You’re not. What you’re doing, rather, is making the naive assumption that you can just rope six or so otherwise-unacquainted people into an interaction together and they’re all going to just hold hands and sing kum-ba-ya. You’re also tempting people to hit “Reply All” in the interest of honoring your decision about who needs to be “in the loop.” But the likelihood of overlooking specific recipients grows exponentially with the number of recipients, and pretty soon somebody is going to say something, or say it in a particular way, that’s meant only for certain people, and somebody’s going to get all bent out of shape.
It never ceases to amaze me how otherwise mature, experienced people, myself included, continue to fall into this well-worn trap.