I feel like there are a lot of really solid, user-focused features that windows phone nailed, that now seem basically lost
One of the Windows Phone's designers had a really insightful article [1] about how they designed windows phone fundamentally for a technology native who doesn't need shitty metaphorical leather textures on his glass screen calendar. It is where the famous movement against skeumorphism started (and thank god for that!). WP was doing flat design before it was cool.
On an old MSDN post (maybe?) I remember reading that the inspiration for the Live Tiles was a user whose central interest was to use the phone as liitle as possible to do his task as quickly as he could before he rapidly replaced the phone back into his pocket to get on with real life. I remember creating a new calendar entry in WP7 took three taps from start to finish, not counting typing out the title. There weren't many other fields to fill besides the Title of the entry and the time/date, and it set a default reminder time of 15 minutes before start time. Perfection. Before facebook and the rest dragged the carpet out from under them, you could get your twitter and facebook feed within the contacts app so you could get it all in one place.
Edited to add: I'm on a bit of a love fest venting session so here's more. WP designers recognized that OLEDs were amazing, so they made the phone White Text on Black from the get go! Thus, great battery life. Further, all the icons were also uniformly coloured, making the whole experience so cohesive. All the apps also looked like all the others, so the learning curve was nonexistent. They recognized how little juice engineers of android could suck out of their bloated multi core processors, so they made their phones work phenomenally well on even shitty, single core processors.
I fucking loved windows phone's early verions. It's a shame they've gone away. But an even bigger shame that they had no confidence in their original designers, and ended up adapting the hideous, appalling, atrocious, fucking retarded hamburger menu before they went away.
[1] http://kruzeniski.com/how-print-design-is-the-future-of-inte...
On early iPhones this was within the reach of the thumb. On later iPhones they introduced the swipe to solve the issue posed by larger screens.
I don't think MS had any confidence in anything they were doing over that time because they never seemed to properly commit to anything. Win Phone could have been bigger and competitive if they'd focused effort on it (marketing - and no not MS's standard marketing 'quality,' app dev funding, etc)
it was somewhat annoying to watch the community go religious on flat design when "Apple invented it"
Seriously though, Windows phones sucked for many reasons (including lack of exposed features in the OS, especially in first versions, and of course lack of apps in general), but UX wasn't one of them. I guess they failed in releasing too much of an MVP, and then not following through properly. I continue to claim that Microsoft had a great UX vision for mobile, and that they're the only one with an OS that can turn tablets from consumption devices into productive tools.
So much flat design fails the "which things are a button" test, WP did it right.
That said, the OS had some flaws on a design level at the start:
Wasted column on the home screen just for the swipe arrow. No notification tray - only live tiles.
Over-restrictive icons made it often hard to tell icons apart. Yes, I get your great design, but pure monochrome silhouettes is a bit too restrictive for app-specific icons.
I loved the idea that a person has a home-screen - that's something that every OS should steal. If I open up a person, I should see every social share they've made, every interaction I've had with them, and every interaction I can start with them. Then I don't care how many messaging apps Google launches, because I don't need to have any messaging apps on my home screen, only my Contacts.