Let me first turn this around. I actually personally tried to interact with the Slack team on how they implemented their XMPP gateway, early on. I pointed out how a relatively small missing protocol feature (server-side group chat bookmarking) was severely impacting the usability of the gateway, as it caused caused you to have to explicitly join the group chat room representing a Slack channel on every client (re)connect. In fact they violated protocol in case a client requested the list of bookmarks, causing clients to hang while connecting. It took them a year to start responding, and the problem was not fixed.

Additionally I had pointed that their statements on XMPP security were factually wrong. No useful response or changes were made.

That all said, I really like a bunch of things about Slack and have repeatedly pointed out in discussions in the XMPP community that there is a lot to be learned from Slack in terms of features (and how they work technically), UI consistency, and usability. As JC points out, this is surprisingly hard to achieve in open source projects. Even harder to pull off for a very diverse community around a set of protocols, rather than a single software product.

There are also things in Slack that I think would be a lot better if they were modelled after recent protocol proposals in XMPP. For example we are working on something called MIX, an evolution on group chat, based on Publish-Subscribe. This allows for orthogonal streams of information bound to a channel, besides just chat and presence, like merge request notifications, Twitter mentions, etc. that could be displayed in a side-bar or ticker, instead of (annoyingly) interleaved with chat messages.

I would have welcomed Slack interacting with the community, but they didn't.