I for one, and I think many others, associate the colourful octothorpe logo with Slack. And also their plaid pattern that is used in many places.
The opinion that I’ve seen best match mine, is John Gruber over at Daring Fireball:
Slack’s old identity had at least three good things going for it: they owned the letter “S” (much like how Netflix owns “N” — something Netflix has doubled-down on as their identity has evolved), they owned the “#” hash mark, and unique among technology companies, they owned plaid. When you saw plaid with those primary colors on a white background, you thought Slack. And plaid isn’t part of any sort of design trend right now. Slack simply owned plaid, to such a degree that Slack company socks — which simply used colors and plaid, no “Slack”, no “S” were necessary to make it instantly obvious these were Slack socks — became coveted swag.
When the Slack iOS app installed on my phone, I instantly forgot where I put it. Sure, it only took a few seconds, but the old logo seemed to draw you in. It had a colourful border, with a bright white background, and the typical “S” that you see with Slack. But now it’s a slightly dull purple, with a generic colourful icon on top. It just blends in.
It feels very much like change for the sake of change, which is exactly what Slack’s blog post said it wasn’t.