- Anyone can be interrupted at any time (to say nothing of @everyone), so it's essentially just an all-day meeting. At least emails could be responded to at relative leisure.
- There are only three notification states: "nothing" (normal icon) "something happened in a non-muted channel" (red dot), and "you were mentioned specifically". There's no way to gauge importance without disrupting your flow. Some pointless cat GIF (why are these being posted on work chat?) is ranked the same as "what should we do next?". Similarly, "@everyone there are donuts in the kitchen, OMG" is ranked the same as "@someone THE SERVER IS ON FIRE".
- Channels are never-ending, so it's relatively impossible to tell where one topic began and another ended. Additionally, multiple conversations can be held at the same time, and it's difficult to tell who's replying to who.
However, I quite like Slack's group private chats. I'd like to see a group-chat solution that promoted those and completely got rid of static channels. Everything's just a private group chat, with all of the people that are needed. When the discussion's done, archive it - it's searchable, of course, but if you need to continue the discussion, make a new one! Maybe everyone in the chat even gets a summary emailed to them that they can search in their email client as well (thus solving the "wait, where did we discuss that" problem).
There are a few other changes I would make - for example, the return key should be newline by default to prevent people from writing
like
this
and instead
putting their thoughts into well-composed
messages