E-Mail is one of those things that falls in the category of "easy to love and easy to hate." While everyone probably has his own list of annoying practices and habits, the Internet is big enough to withstand one more list, so I'll add my two cents.
The subject line is the most important component of an e-mail, and most people don't seem to have a clue about how to conduct themselves in a subject line. A useful subject line really isn't that hard to create but can make a world of difference in how an email is received. Saucci's Three "E's" for a good subject line are simple: Exists, Essentials, Exits.
The first rule for a good subject line is that it must EXIST. Even in this day when so many spam filters simply mark as junk, delete, or even bounce emails without a subject line, many folks just skip a subject line as if it is some sort of extraneous fluff and they are too busy to waste time on such trivia. Don't go that route. Even if your naked email gets past all the filters, a missing subject line is a serious lacuna. It might take all of a few seconds to type something useful in there, so how about helping your recipient by putting something in there?
The second rule is "cover the ESSENTIALS." You may have heard the phrase "money words." That's what you need in the subject line. Summarize the whole email in half a line. Again, it really isn't that hard. Don't treat the subject line like a pain-in-the-neck detail. Some people just type their name in there again, as if they are annoyed that they have to type anything at all there, but they know about the first "E" of subject lines and just put something-- anything-- there. As an IT professional, I often get subject lines of "Computer," or "Problem," or "Hi Andrew," all of which are useless to me as I scan the list of emails trying to see which is the most important and should be put at the top of my usually lengthy to-do list-- and which ones should be summarily deleted or sent to the bottom. A subject such as "My printer has a paper jam" or "Need signature on agreement by Tuesday" lets me know where to put something in the list.
The third rule for subject lines is also simple: Summarize succinctly and EXIT. Some people start typing their whole message in the subject line as if it were a text message, just because the inane email client and the relevant standards allow it. Please remember: E-Mail is not a text message. If your subject line runs more than half a line, it has overstayed its welcome. Even the very same email clients that allow one to type a novel in a subject line will be forced to truncate the subject when displaying the inbox. If the money words are not present in the first half line, they may not even be visible-- and don't assume that everyone has his e-mail client set up the way yours might be, with a preview pane enabled. Mine is disabled, as are many others. Put the body of the message where it belongs-- in the body, not the subject line. They don't print novels all on the front cover of a book and leave the other 200 pages blank.
With the three "E's" covered, what else is annoying? Lots of people will dig up the last e-mail a correspondent sent and reply to that-- and leave the subject unchanged! One person did that to me this week and I wound up spending an hour investigating a problem that did not exist. One could argue that I should have actually read the e-mail, but the subject line appeared to say it all, while in reality it said it all wrong, and just like the people who write to me, I am an imperfect human being. Take the extra moment to revise subject lines as necessary. That applies as well when a conversation drifts into something else-- change the subject line when the original topic is no longer relevant. I frequently do this so that I can find emails later if I need to go back to them. Finding an e-mail about a paper jam is lots easier if the subject is "Paper Jam," than if it is in a discussion titled, "No Internet."
Related to the need for good subject lines is dealing with multiple disparate topics in the same e-mail. While a punch list is sometimes useful and can be summarized in the half a line a good subject takes, generally I prefer that different topics be separated into different discussions, each with a unique subject line. I may need to forward some items on a list for action by others, or I can take some items on a list and file them in my "Finished Items" folder. When the discussions are separate, that's easy. When they are intertwined, it's like untwisting a pile of spaghetti.
Some people mark all their emails with the red "Important" exclamation point. The more often it is used, the less priority it gets. Use it sparingly, if at all. Similarly, some people put the word "IMPORTANT" or "URGENT" in the subject line almost as a matter of habit. Practically every e-mail I get in the office is urgent or important to someone, so it doesn't really get it any special treatment; don't waste the effort or the space. Save the subject line for money words and I'll decide where they fit on the to-do list.
Finally, much of the foregoing applies to business correspondence, though it applies to some extent to personal correspondence. In my personal mailbox, some things are important to me just because they came from a friend, and I don't give a hoot if my friend types a subject of "Hi, Andrew" or if she follows all the rules. Friendship does have its privileges!