I think the Windows Phone OS provides an outstanding experience. There are little features like 'Quiet Hours' that I don't think exist on iOS or android. Also, the hardware (Nokia/Microsoft) is bulletproof. I'm a little nervous about the
relative lack of durability of my soon-to-be-delivered 6S+.
I've never had an iPhone. Years ago, I flashed android on to my HTC Touch Pro II, but it was terrible. I've been with Windows Phone since day one. In the beginning with Windows Phone 7, the app gap was understandable and forgivable; I could easily live without apps like Words With Friends. With Windows Phone 8, the app gap really started to close, but recently, major 1st party developers have started pulling back. For example Chase pulled its banking app, so I can no longer deposit checks with my phone. I'm sure other WP fans might argue otherwise, but for me the app gap has widened. I am also very interested in mobile payment. Softcard was supposed to be the answer, but google screwed that up for me. Actually, google has really screwed up a lot for Windows Phone, and as such, I go out of my way to not use their services. Also, T-Mobile seems to have gone out of their way to inhibit Windows Phone; I'd switch to AT&T if it wasn't so expensive.
So to answer your question, I'm really just tired of waiting for Windows Phone to improve. I hope it succeeds! All consumers benefit from strong competition. This may fall on deaf ears here, but Windows Phone has provided some innovations that have been copied/replicated in iOS and android. Also, Microsoft has recently released nearly all of their apps that I care about for iOS. So, while the UI might be a little bland, I know the iPhone will give me access to all apps and features available on a smart phone.
tl;dr: app gap (and to a lesser degree, missing features)
PS - I know some of you are staunch Apple fans. I am not trying to troll or looking to start and argument with this post. TechDiva asked me a question, so I answered.