I can understand why they wouldn't support Linux/Blackberry/WP. As a Linux user I'm used to that. But not having windows support? Really?
For Windows probably will be the Browser extension/addon which according to other comments will be using WebRTC.
They are releasing a full Windows client soon.
I don't get why they decided an OS X app was more important to release than a browser app... A browser app reaches a way bigger market share than an OS X one.
The Mac app is also pretty awkward on non-Yosemite computers; a very broken UI.
And no love for Sailfish OS as usual :-(
On the other hand, if it’s any good, there’ll be an open Sailfish-only app for it soon?
I doubt it. The problems with apps in Sailfish is that their appstore doesn't seem to support any payment for apps, so all the apps need to be free. I think this is maybe not giving incentive for people to actively develop on that platform unfortunately.
But perhaps someone comes up with a free app, you never know...