It turns out that, for most people (in a corporate setting at least--not sure about college campuses), the problem is that they don't even know how many people they are sending their message to; distribution groups hide how many people are truly on the To line.
We introduced a feature in Exchange 2010 specifically to combat this (as a part of a broader feature called MailTips). In Outlook and OWA, it will warn you as you're composing your message of exactly how many people will get the message.
Blog post introducing the feature (with some really old screenshots):
I was there when this was happening and realized exactly what was going on at the time. Still, it was very very hard not to respond with a 'Stop replying all' type message. There is something about these things that makes it hard to just sit it out and shut up (at least for me).
The list had a broad section of the company. It wasn't surprising to see some of the less computer savvy users responding. I was surprised though when I saw engineers down the hall responding.
When the blog post author (Larry Osterman) states that a quarter of 100,000 is 13,000, and that 10% of 13,000 is 130... I start to question if anything the author writes is in any way reflective of what actually occurred.
http://blogs.technet.com/b/exchange/archive/2004/04/08/10962...