A little while ago, Microsoft blogger Raymond Chen suggested that you should use a subject line that's meaningful to the recipient when you write email. I strongly agree, and have more to say about that here. I also like Raymond's blog a lot, and want to link to it here. So I have.
My first comment is that you should use a subject line that's meaningful to everyone — to all recipients, if there are more than one, including everyone on the CC list, and to you, the sender. My favourite example in Raymond's list (because I've gotten these too) is “URGENT URGENT URGENT”. Remember that the recipient(s) will lose this in the mess, assuming it gets through their spam filters (see below). But remember also that if they do manage to respond, their responses will have a subject of “Re: URGENT URGENT URGENT”, and that won't be meaningful to you, either.
This is especially true for meeting invitations. At the office, it's common for executives to have their administrative assistants manage their calendars. The assistant will set up a meeting with me, and will send me an invitation on our group calendar system. The invitation will often have a subject like, “Meet with Barry.” That may be fine when it's on the executive's calendar, but it doesn't do much good when I accept the invitation and it's added to my calendar that way. Better would be “Meeting, Ellen and Barry.” Better still, “Discuss antispam project, Ellen and Barry.”
Do not ever, ever, ever leave the subject blank. Some systems will send the message with an empty subject. Some will fill in "No subject". Either way, it's not useful and is very likely to get your message eaten by spam filters. Even for social mail, fill in a sensible subject.
A friend recently asked me, “What does 'Kein Betreff' mean?” Ha. It's German for... “No subject”, of course. And my friend was wondering what the correspondent was asking, and thought the subject was significant.
Now that you're filling in a subject, don't just put noise in there. I have a friend who likes to use “from Barbara”. I know who it's from; the email already tells me that. Also, that's another incantation that's likely to run afoul of the spam filters. Try to put something there that says something about the content of the message, even for social mail. If you're talking about a restaurant you went to, put the name of the restaurant, or something about the food you ate. “I love salmon!” is a much better subject line than “Saturday” (but even the latter is better than nothing at all).
For business mail, this is critical. The more information you can put succinctly into the subject the better (but avoid being over-long; more than 40 or 50 characters isn't useful either). “A question” isn't useful. “A question about DKIM” is better. “A question about DKIM canonicalization” is even better. Intelligible abbreviations are fine, to keep it short. But the point is that when I'm looking through my inbox, I want to be able to identify the messages there, find which one I'm looking for, know which one I want to take action on.
Remember that spam is a huge problem and that some spam filters are quite strict. Avoid things that make your messages look spammier than necessary. Of course, anything can be mis-classified, and modern antispam software considers much more than just the subject, but there are some things in the subject line that are red flags and you can probably figure out what they are. Blank subjects. “Hi!” “Help!” “Urgent!” Things like that. Also, strings of exclamation points (“!!!!!”) or question marks (“?????”) are suspicious. Avoid them; they don't help the recipient, and they only make the subject line longer for no good reason.
In general, keep in mind that you might get only three messages a day and not care what the subject lines say, but that your recipients might get dozens or even hundreds of messages a day and be lost without sensible subject lines.