Mostly a pet peeve, I guess. The subject should describe what the message is about, the body is the message. After opening the message, I automatically look down to read or skim the body. When there's nothing there, it annoys me to no end.

Worst case… at least repeat the subject in the body. Also it annoys me to no end when messages aren't formulated in correct english. (Text-speak is a big no-no.)