I never had issues with battery life. And living in a land of no data caps on most subscriptions, I actually hated the fact that early versions refused to download e.g. some updates over mobile data.
I mostly liked the user interface, too, and the virtual keyboard was the best I've used. But for me, there was one huge issue: lack of apps. Even things like Whatsapp came really late to the platform and lacked features. I think it is because the SDK was completely different from other phones, and the optios to share code between Windows and Android phones were limited. The way multitasking was implemented also made writing apps quite difficult. And the SDK was very limited, most anything beyond basic components you had to write yourself or use 3rd party ones. Including even some widgets you could find on the apps shipped as a part of the OS.
There was a lot of potential, but Microsoft was too arrogant in making their own walled garden (even at first charging obscene amounts of money for the SDK!) and focused on rewrite after rewrite instead of focusing on making things better.
Dynamically binding, you realize the magic. Statically binding, you see only the hierarchy.