Will there be an API for 3rd-party clients (perhaps ones that become officially sanctioned ones at some point), so that we can build a proper desktop client for those that don't run Macs?
Although it's not quite the same as a native app, we already have built in support for desktop notifications on all browsers that support them, including some older WebKits.
There are still a great many organizations that are using XP. It's a reliable and battle tested OS. Ignoring these users wouldn't be a good idea, I don't think.