For as long as I can remember it has been customary, in replies to messages in email and on Usenet, to quote portions of the original message, usually by preceeding the quoted lines with “>”, like so:
Joe User says: > I've compiled the compress program, but I can't find the > source for uncompress. They're the same program. Just make a link from uncompress to compress.
The purpose of the quotation is to provide context for the reader of the reply.
Sometimes a reply includes a complete copy of the original message. In email, this is only marginally helpful, because it provides only very course-grain context, not much beyond what is provided by the Subject field. It also clutters the recipient's saved mail archive with redundant copies, making searches take longer and return too many matches.
Imagine if everyone always quoted the entire message: A transcript of a conversation containing, say, a dozen messages would actually contain twelve copies of the first message, eleven copies of the second, and so on (that's 78 total in this case, O(n2) in general).
I have two recommendations for making quotations more helpful and less wasteful.
Intersperse the responses among the quoted material. This helps the reader easily see which parts of the response correspond to which parts of the original. Put each response after the corresponding quotation, not before. If the reader doesn't need the context, the quotation is equally easy to ignore no matter where it appears; but if the reader does need the context, you want the message to make sense when read sequentially.
Delete unnecessary quoted material. In many replies, the vast majority of the original message is unnecessary. Deleting unneeded portions not only reduces waste, but also helps the reader see exactly what the responses are responding to.
If someone is unwilling to follow these suggestions, I would much prefer that s/he quote nothing at all, rather than quote the entire original message in one block. But I think that by following these suggestions, people can significantly improve the effectiveness (and decrease the cost) of their textual conversations.
If you agree with me, please refer people to this page. The practice of quoting entire messages has unfortunately become much more common than it used to be.
Here is what I do when composing a reply. I begin with the entire quoted original message, and start scanning through it from the top. Everything that I'm not responding to, I delete. Whenever I get to something I want to respond to, I skip past it and insert my response, then go back to deleting things. By the time I get to the bottom, I have complied with the above recommendations. I may then insert additional material, and sometimes I reorder the responses if that makes the message easier to follow.
Note to mail program authors: In light of the above procedure, the best initial placement of the cursor to encourage good quotation style is on the first quoted line (not to be confused with the attribution line). The signature, if there is one, should appear after the quotation. I suspect that mail programs that put the cursor and/or signature in other places by default are partly to blame for the prevalence of bad quotation style.