I have three questions on this topic:
1) The new Exchange Permissions system (circa Office XP) has a "Do Not Forward" option but not a "Do Not Reply All" one. In my opinion the latter would be infinitely more useful at curbing embarassment, confusion and unnecessary churn in day-to-day communications. Please add it! :-)
2) Why can't Outlook warn me, e.g. "This message will be sent to more than 1000 users. Are you sure you want to continue?" when I click send? It would make a lot of people think twice. (Don't answer that, I know *why*... nested DLs and private members, etc. I still want the feature though. You're Microsoft, make it happen).
3) Why weren't those distribution lists locked down so that only their owners could send mail to them? This is a feature in Exchange, right? And is it also a feature that only members of a DL can send mail to it? I'd hope so, but too often I see external Internet email coming to internal DLs. It seems like this functionality should be off by default.