Ah, I should have known. My reasons, in no particular order:
First, it was Yet Another Communications Channel to Check. You know, email, phone, voicemail, meetings, standups, calendar appointments, and so on. It was one more place I had to scan through on an event loop. I would pay more attention to Slack only to miss something somewhere else. Where was communication happening? Anywhere. And Slack made it worse because we had a skillion channels proliferate, so it wasn't as if Slack were one email inbox, it was dozens.
Second, Agile Cosplayer #1 was busy posting random things like his Spotify playlist for the day (I don't have Spotify) and other stuff. You can't even /ignore like in IRC.
It seemed especially silly when all of us were within ten feet of each other.
Partially my fault: I was late to the smartphone game and have no idea what roughly half the emojis meant, and so I was missing whatever emotional tenor of a lot of messages.
Probably the worst part was that the expectation was that always had one eye on Slack instead of, you know, focusing on programming. There was no "getting back to" someone on it. It was expected I would have it on my smartphone (newly purchased) and have it just running all the freakin' time.
Most of this comes down to local implementation, hence I can point the finger quite a bit at Agile Cosplay, but boy oh boy, does Slack enable and encourage some lousy local implementations.