The UI was very good. The tiles didn't work on the desktop (Windows 8) but it make a lot of sense on the phone. Every touchable element was large enough that you'd never miss. The UI itself ensured a very consistent experience. I assume that you could do more custom apps, but most apps I used conformed to the general look an feel of the operating system.

One thing I don't think enough people mentions is the messaging app. It integrated with Facebook, before Facebook Messenger. This might not be a big thing today, with people leaving Facebook, but it was absolutely brilliant back then. Basically like iMessage, but with Facebook as the non-SMS protocol. I think it may also have integrated with LinkedIn, I don't recall, but it was suppose to just merge all your contacts into one messaging app.

As much as I dislike Windows on the desktop, Windows Phone made perfect sense. The UI was completely tailored to the touchscreen and was so easy it use and navigate. It's a real shame it didn't take of. In some sense Microsoft should have treated it more like Windows on the desktop and aggressively offered it to OEMs.