And Wave.
Spereately, I think it's hilarious that many websites pester hard for emails, making them effectively pamphlets for their email content. If only there was a way that I could subscribe (and unsubscribe) to a stream of short notifications from content creators and be confident that I see all of them in a timeline order. Like what Twitter used to be before they screwed with the timeline. They're going to lose to email, and not for being ahead of their time.
Eh, the biggest problem with using Wave was that it was only just barely usable on top-tier hardware of the time. So, for most people, it was impossible to run on their own hardware (especially on their laptops).
I think that if the performance hadn't been an issue (ie, the average hardware in 2009 had been capable of what it is in 2016), it could actually have been comparably successful to Google Docs or Google Calendar.
Google Docs and Google Calendar are very restricted in comparison. Gmail is as well. And search - type some words get an ordered list. If you don't get what you want try some different words. Pretty damn simple to use and explain.
The ideas behind Slack are very old. Doesn't stop Slack from being incredibly valuable as both a product and a company.
In many ways, Slack is trying to become what Google Wave tried to be at the beginning (a tool for real-time communication with, not just in chat/text form, but with other embedded forms of media as well).
I was less excited when I found Wave was its own product, rather than a Gmail integration. Putting the widgets inside the same email inbox you have open for hours each day would be the best way to get people to actually use it.