Before I start, first off, I like Windows 8 as a concept, but hate it's execution. I also hate the design of iOS 7 and the blind application of flat design.
I think Windows Phone's UI is just fucking terrible:
This guy explains it way better than I can:
Yes, I have. In fact, I did graphic design for Nokia until very recently.
The problem with WP is that visual consistency doesn't equal good usability. Usability should always trump visuals in the pecking order. Yes, WP looks consistent, but the design language and UX are riddled with flaws. I'm going to list a few.
First and foremost: there's no branching of users. Whether you're a power user, a 90-year-old granny, a toddler or a Brooklyn hipster, you get the same UX. For proper usability, the UI should bend to the needs of all user groups regardless of their skill level. If you want to implement a one-size-fits-all solution, usability is critical. WP, however, skimps on usability to maintain visual consistency, and doing that while trying to implement an OSFA solution means the result is confusing for the toddler and frustrating for the power user.
Secondly: the most used functions should be always accessible, period. When a user picks up a phone with a specific task in mind, his path to complete that task should be as optimized as possible. That's why iOS has a quick access dock as well as most Android phones. WP, again, does not to keep a consistent look. You do get live tiles, but no dock, which means that if you're using more than a handful of your phone's functions you need to scroll down, making completing critical tasks slower.
Thirdly (and this is my favorite pet peeve): the WP design language is directly opposite to Gestalt rules. The UI should use color, weight and visual cues to guide the eye and to assert a visual hierarchy. You need to be able to form an overview and the location of the most critical functions of the phone with a quick glance. This is usually done using color, continuity, contrast, focal points etc. WP disregards all of this. If you look at the live tiles, there are useless animations, complete disregard of visual cues and the focal points are all over the place. Why is the Red Bull app colored, thus stealing my attention, while the more important phone and message icons are not? Why is the Xbox Live tile green by default, making it the only one to stand out? Is the UI suggesting it's really the most vital app of my phone? Why are the tiles moving without input, making it exceptionally difficult to form an overview of what I'm seeing? Why are my eyes being constantly guided to non-essential parts of the UI? Thanks to these things, the UI is crowded and it's difficult to form a visual hierarchy of what I'm seeing. In most cases there's also no distinction between call to action elements and plain text, making guiding the user through visual cues difficult.
Fourthly: no folders. Only an alphabetical app list. For example, if you need to access multiple apps you often use together quickly, you can't have them sitting next to each other on a folder. You're forced to scroll up and down the list every time. Instead of having connected apps next to each other, you're forced to make the connections in your head, which is cumbersome and needlessly taxing, once again all because of consistency.
I'm going to save you some time and stop here. I'd have material for an average length thesis on exactly why the WP design is woefully inadequate and why it is clashing against hundreds of years of visual design theory, but let's save some time and just say it sucks with the force of a thousand suns.
