Working in IT and administrering phones for the company, I had plenty of opportunity to work with Androids and iPhones, but hadn't worked yet with Windows since the Win Mobile days. My current personal phone was a Moto Electrify 2 (Droid Razr clone) on US Cellular. Because of problems with their data service in our area, we switched to Verizon and I took the opportunity to pick up a Lumia 928. Just prior to that, I ordered a Lumia 822 to have as a spare at work - and also to take on a trip and ensure I could live with Win Phone as a daily driver.
Needless to say, that trip convinced me. I found myself reaching for the 822 rather than my old phone out of instinct and not out of forcing myself to use it. And trust me, I could operate my previous phone in the pitch dark out of familiarity.
In short, the combination of the UI - it can be as simple and basic or all encompassing as you want it - and the Nokia hardware hooked me.
My parents will be getting WP when they're due for upgrades for the simplicity aspect. I have that work 822 to use for evangelizing. As mentioned by other posters - Google is beginning to wall off Android. I found Google Now annoyingly intrusive and invasive. Never got a feel good vibe from the iPhone that matched its hype - especially using iTunes/iCloud in a Windows desktop world. It may "just work" - but when it doesn't it royally screws you - and more importantly - your data.