Without having the time to read every reply in this thread, I hope I'm not repeating any gripes/features/etc.
I have a HTC Trophy running WP7.8 from Verizon (one of my gripes I'll list later). I've been running it for 2 years, and I'm seriuosly considering going to a Samsung Galaxy S4 for my next phone. I really like my Windows phone, but there are a few things that I need. It's not that I dislike the Windows Phone, but there are specific apps that I need, and only exist on Android or iOS market (I'll never go to Apple though).
The app store is taking too long to develop. Like I said before I need some specific apps to perform certain functions for my job, and I really need to get them. It seems some companies have staffed developers who have gotten used to only developing for Android/iOS, or don't like WP. I could possibly write an app for WP, but don't have the time to do it. I wish Microsoft could develop more partnerships, or offer incentives to develop apps, I'm not sure what the problem is.
I've grown to like the metro style a little, but I'm still not sold on it. What would help is some more customization of it. I wish you could make some icons different colors, or group icons, and color the groups...something to group certain icons (i.e. messaging apps are blue, games are red, weather apps are orange, etc.) Everything the same color makes it hard to find some apps quickly. Also some apps (even native ones) don't allow for rotation, which is annoying.
My biggest gripe/request with the music player is the playlists. I wish you could create/modify playlists easier. Creating one from the now playing group is a small start, but if you accidentally add a song you don't want in the playlist, the only way to remove it from "Now Playing" (that i've found) is to completely restart the phone, and start your list over (that SUCKS). Add/Remove songs from playlist, and auto-playlists would be a huge feature for me.
One of my most painful problems with WP are updates. This may have been a problem more with Verizon, than Microsoft, but getting the 7.8 update was the most frustrating expereince I've had with a mobile phone. I waited and waited for WP7.8, with no word from Verizon, I read forum after forum, and some people were getting it, some weren't. I waited for what seemed like a couple of months. Verizon would make absolutely no comment on the update. I finally found a way to do it with a hack, and updated it myself (I'm not sure I would have ever gotten the update if I was still waiting). I also owned a Samsung Omnia many moons ago, and had the same problem with it. Windows Phones never seem to be developed/built to accept a newer mobile OS. It's always the same excuse, the hardware is not acceptable for the new OS. Well then, either develop the hardware to have a longer lifecycle, or develop the software smarter!
Would love a way to create volume profiles, and have shortcuts to apply them quickly. I listen to music in a few of different scenarios, and sometimes it requires different volume levels, and/or different sound enhancer settings. Also since the 7.8 update, my bluetooth ear phones are no longer detected as headphones under sound enhancer, and I cannot apply any equalizer setting to them.
These are a few I could think of....