One of my pet email peeves, after useless, annoying, irrelevant and lengthy signature files that get added to every email is/are wrong or blank subject lines.
Blank subject lines being a result of what exactly? It can't be laziness as it really can't take that much more effort to type at least one relevant word in the subject field. Surely not. My standard reply to this is to change the subject line to "Subject lines don't bite you know"
What definitely is laziness is digging up a previous email (often this is a reply-to-all group email) and using the existing address list to reply with something new, arbitrary, or unrelated. Grrr. I try change the subject line, but on a group-reply-to-all-fest, this doesn't help all that much.
Now I sort my emails by thread or conversation. This is pretty effective way of grouping things together for easy findability or deleting a whole discussion when it has run it's course.
However you lazy subject liners screw that all up. I end up wtih half my inbox as a group of disparate unrelated mails titled "RE:" and "FW:" and the other half with mails that have a subject line about something 6 weeks ago, but content about something happening next Friday.
Sigh. Really people. Subject lines aren't that hard to do. Help me help you.

1. matthew
I really love in Gmail when you get a message with no subject, and Gmail cunningly titles said mail ‘message with no subject’, just in case you didn’t realise that in the first place. And Gmail’s handy conversation view goes slightly awry when there are no subject lines.
On the subject of peeves, it’s all very well sending/forwarding jokes (I like a good joke in my mail as much as the next man), but I tend to edit out the 4 or 5 ‘Fw’s in the subject line, and the 5,000 email addresses in the body of the message that come before the joke, before I send/forward a joke I have recieved. It keeps the joke funny, you see. I just wish others would do the same…
2. annie
On the other hand, subject lines can be too tempting. For example, I’m dying to open the email I got from Brianna Healey headed “Take just a candy and become ready for 36 hours of love” , though it can only disappoint.
3. Gordon
I’m a big fun of subject ONLY emails. Rather than have to open an email to read “Yes, will meet at 8” just bung it in the subject line!
4. Adrian
Yes, likewise. It’s all about efficiency. Except for URLs. Don’t put URLs as subject lines, it’s messy.
5. Lyle
Ah, now I hate the subject-only emails - they do my head in.
We’ve got one person in the office who puts as much text as possible in the subject line, and it’s annoying as chuff.
It’s simple - subject for what you’re mailing about, and main bit for (gasp!) the email itself.
Rocket science it ain’t.
Well, unless you work for NASA, in which case it might be.