Here's one that should affect everyone who's had to work around email. In probably every company that relies on email, you'll see email sent out to the masses -- whether it's 5 or 500,000 people isn't the issue. Every once in a while, you'll get someone who decides to Reply All and say something that you will most likely not care about. Then, in what seems to be a snowball effect, others start doing the same. Then, you start getting the people who are just as annoyed with the junk email as you are and Reply All with a message of "Unsubscribe" or something similar to mark their disinterest and others' disrespect of their email inbox. The problem just gets worse from there. I think I speak for everyone when I say that there has got to be a work-around to this... besides weeding out the idiots.
One other scenario where I think the Reply All comes into play, but not necessarily as drastic of an annoyance, is when you BCC someone. As I'm sure you know, when you BCC someone on an email, those who receive the email don't know you sent it to the BCC'er(s), yet they still have the ability to Reply All. This can raise tensions, depending on the context. So, obviously, limiting the Reply All feature here would also be a good thing.
Basically, I'm thinking that Outlook needs an option to specify when the Reply All feature is available. This option alone could save a lot of pain. Obviously, someone can still do a manual Reply All, but that would help ensure that the user truly wants to reply to everyone, instead of not realizing who they're sending the reply to (namely, senior execs who you're not on a first name, fart joke basis with). I'd hate for there to be a dialog box that asks if a user is sure they want to Reply All, because I know a lot of people hate dialog boxes. But something does need to change.
The BCC scenario actually has 2 parts, in my mind. First and foremost, the receiver needs to know that they have been BCC'ed. If they don't know, then they are more likely to Reply All. Obviously, the second part is the actual Reply All. Perhaps a Fwd to All option might be used in the BCC scenario to signify that this person is a masked part of the conversation; however, that could still cause the same issues. My hopes for that would be that seeing the different text on the button would make the user realize what they're doing.
Honestly, tho, I think that any effort made to hide the capability would eventually come down to the user getting an option whether or not they want to Reply All or at least whether to respect the sender's choice to show/hide the feature. This is somewhat like the option to be notified when someone reads your email. I believe this was automatic in Outlook 2000; however, in Outlook 2002, the user received an option as to whether to send that notification back or not. Personally, I like that. I'm assuming any attempt to limit Reply All would come down to the same decision. Even so, at least that's something!
Edit: Outstanding news... this is already in Outlook 2007. The feature may be in earlier versions, but I'm not sure. In Outlook 2007, when you're creating a new message, click the Office button (Alt+F) and, under Permission submenu, check the Do Not Reply All option. It's a beautiful thing.