I don't even care about other features. The tiling home screen of the OS was really nice to use and when used properly by the apps could result in a "live" OS unlike the iconographic interfaces of iOS and Android. The homescreen was also old-age friendly and really a pleasure to use.
The OS ram like really smooth on 512mb RAM, unlike their counterpart android phones which were struggling back then with 2-3GB RAM.
The lumias themselves had a ton of useful features like tap to wake etc, which didn't consume much battery and in general the Nokia cameras were top notch for the time.
Basically, the OS got killed because of a chicken and egg problem with the apps, and the OS being from Microsoft, got a death knell because of the reputation. Also for some fucking reason, Microsoft decided that the already low userbase WP7s were to be depreciated rather than provide an upgrade path fo WP8 and WP10.
Not opensource, but i love juicessh. I keep coming back to it all the time.