I thought I would share a little pet peeve of mine that many people do not, apparently, agree with: an email subject should not contain unique information.
The way I expect and write emails, the body of the email should be self-contained and should represent the whole of the core message. The rest of it is just an "envelope" that should be able to be discarded, more or less. The purpose of the subject line is to indicate what the contents of the message will contain, but by definition, it should not contain unique information that is not already contained in the body.
I know that many people do not follow this advice in one direction or the other. Some people often put the entire semantic contents of the message into the subject. Those who do will sometimes helpfully include EOM at the end of the subject line. That's marginally tolerable provided that the message is truly short. Worse, though, is when people include some information in the subject line, but most of the information in the body text. This causes me to often need to read the body and then get confused about missing information, only to discover it in the subject line. This is inefficient, hard to consume, and not very nice.
Then there are people who put no meaningful information at all in the subject line, and rather use them as placeholders for emojis, or their mood, or some "greeting" or another, like it was the salutation of a message, with no indication as to the contents of the message nor any hint about when or if the message is worth opening at a given time. This means I actually have to navigate to the message and open it in order to decide whether the message was worth opening at this time. That's just...rude.
However, I am rather surprised by the number of people who do not follow this advice. I have even found myself sometimes not following it myself because it seems like people sometimes don't want me to be thorough in my communications (literally). To me, all of this represents a situation in which the convenience of the sender is prioritized above the capacity of the reader to be maximally equipped to efficiently handle that email with the minimal amount of further communication. It's, IMO, a violation of traditional concepts of etiquette, but I recognize that not everyone will agree with me on this.
Nonetheless, I just thought I would throw this out there, not because I expect anyone to necessarily change their behavior regarding this, but to vent my own feelings on the matter, both as a writer and a reader of emails.