Engineers grew up on chat (IRC, AIM/MSN/ICQ, Jabber/XMPP, etc.) and have opinions about what it should look and feel like. When a company raises billions for a product that feels like using dinky Fisher-Price software that you could have written better and gotten rich off instead, and moreover when their product is forced upon you by your workplace and it becomes a major distraction and headache...

Ire, jealousy, hate... There are a lot of bad feelings towards Slack.

These are not necessarily my feelings, but I'd be lying if I said I didn't dislike them just a little.