[otterley]

I miss the octothorpe (#) -- it was a clever reminder of Slack's origins in IRC, where channel names start with the same symbol.

(Technically, in IRC, they could be prefixed with an ampersand (&) as well, but nobody ever did that. Great for making super-secret channels, though.)

[iaabtpbtpnn]

In case anyone's wondering about the difference, a &channel is local to the IRC server it's created on, while a #channel is globally usable across the IRC network.

[omouse]

This was the cool part as others have noted. For the techies and nerds, # = IRC and it's like "cool, a nod to tech we know and love and now we have a replacement for". For the Twitter or Facebook generation it's a hashtag, "cool, a nod to something I use everyday and love using."

[spike021]

Or channel names prefixed with double ##'s.