A few things, but I think the best part was opening the Settings for the first time and seeing a clean, elegant design which showed me all the information in one glance. Great use of the Segoe font at last, and proper balance of elements throughout the page. That was the moment that first made me positive about the future of this OS, knowing that this kind of design would form the basis of other apps to come, and that the most attractive parts I've seen of the desktop W10 were going to be echoed in WP10.