I loved how WP took the "keys" to design away from the apps. They created a very simple, restrictive design language and locked apps into it. You want color? It's a button. You want to put color on a non-button? Piss off. Colored rectangle = button.

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.