Holy cow, I wrote a book!
This applies to email, but it applies even more to meetings and appointments, which are typically arranged via email or similar workflow.
It may seem obvious at the time, but messages with pronouns like me or you in the subject line tend to be harder to understand when you come back to the message a little while later.
I was reminded of this by a colleague who received a meeting reminder alert that simply read:
Title: One-on-one chat Start time: May 9, 2007 4:00 PM Location: my office
Whose office is my office?
This is a generalization of choosing a subject line that is meaningful to the recipient. You want a subject line that is meaningful to everybody involved.
And here's the round-up of some unhelpful or rude subject lines I've seen in the past five months: