I'm an ex N9 user and currently WP8 user. I am lucky that I don't use a lot of apps or I play almost no game on my phone. Most important things for me are email, camera and browsing.

But still, I use popular apps like Facebook, Twitter, Foursquare, Youtube, Whatsapp, Viber, Skype and Shazam. I'm sure there will be great apps eventually for open-api apps (actually probably better than the crappy Facebook app on WP8) but unfortunately the last 4 apps are closed API. Also there are huge amount of high quality camera apps on WP8 right now.

My N9 had better social integration like GTalk and Facebook chat, I'm sure Jolla will be like that as well. Also being open source and more end-user oriented it will be updated faster than WP8 (we still don't have a lot of cool features of N9, like glance screen, flip to silence, double tap to wake etc...).

So I haven't decided yet. I loved my N9 and still miss it but WP8 is right now the solid third platform with almost all the apps I use. I'll most probably watch some review videos on Youtube after it is released and decide.