There are many things, but I'll go with one that hasn't been mentioned.
I love being able to email an actual Word or PDF file from the phone, or more specifically from the Office app, rather than a link. The ability to share an actual file on the go is a major plus for me. Previously on an iPhone I was only able to share a link from Dropbox, and occasionally this proved troublesome and confusing to the receiving party. On WP I am able to send the actual file.