[depaya]

I can tell you why it bothers me personally. As soon as I see "Hey" my attention has been captured by the sender. I now have to sit there and wait for them to type out everything else. Or they sit there and wait for me to respond before they send any follow up. After waiting awhile maybe I learn that what they are sending is not time urgent, but too late my attention has already been completely removed from what I was working on.

The root of the problem is some people see Slack as more synchronous than others. I personally see Slack as more async (more synchronous than an email, but less than a phone call). I screen messages as they come in and respond in a way that makes sense to me. Likewise if I send somebody a message I don't expect an immediate response. Somebody simply sending "Hey" forces the conversation to be synchronous.