Honorable mentions go out to FirefoxOS (maybe as viable as WP?), Tizen (OK, maybe that will be in some cars or TVs), and Maemo/Meego/Sailfish (oh how I wish you would have been successful!).
Edit: I guess I forgot about Pocket PC. Maybe MSFT can be considered to be an early starter that lost the picture?
The app selection is really, really surprisingly bad. Not just for the popular missing apps (snapchat, etc) but just about anything has a ton of dreck, and if you're lucky one or two functional ones.
I'm pretty surprised at how badly Microsoft has courted app developers on this platform.
Business. If I recall, the NYPD bought a huge load of them for deployment with officers a while back. Is there a viable path to consumer daylight? Nah. But PWA stuff would at least allow business users to get something.
However given the current announcements, me and others will have to focus only on Android, it seems.