I'm currently using a Nokia 808 on Symbian. My wife has a Lumia 521, and I've used it a lot to see if I like WP. I've owned an Android in the past.
I moved from Android to Nokia (N82, N8, now 808) because of the fantastic cameras with xenon flashes. On the N8 and 808, I've really appreciated the rock-solid and fast multitasking OS. It's like Android without the lags. Sadly, Nokia let Symbian die.
The main real reason I am considering a Windows Phone is for the quality of the Nokia cameras. No Android or iOS phone has a camera that comes close to the 1020 or 808. Unfortunately, that means I'd have to adopt WP. After using my wife's 521 over the past month, I thought I could deal nicely with WP. I like the aesthetics and most of the apps I want exist already. But then I started doing more research into the type of usage I'd want from my phone (and which I can't test on my wife's phone, since I'd ruin her set up.)
The problem for me is that forum after forum lists the many ways that the WP OS is buggy or is missing core functionality taken for granted on other OSs.
Key bugs:
1. Duplication of music files and amazing hurdles to put music onto your phone's memory and SD card. This is a complete deal breaker for me, as I'll have around 80GB of music on the phone and use the phone as much as an MP3 player as a communications device.
2. Duplication of photo images.
Key deficiencies:
1. No sound profiles.
2. No way to automate sound profiles (I need my phone to automatically go silent and then back to normal at different times each day, according to my weekly schedule).
3. No differentiation between sound volumes (I know this should be coming in 8.1).
4. Too much screen real estate given to overly large Metro titles. I like the concept, but wish the title fonts were smaller. Often on my wife's phone I feel claustrophobic because there isn't enough real estate given over to content. I seem endlessly to be scrolling.
5. IE is awful. I dread it when WP apps launch IE to show their content.
6. WP seems to be trying to be a gamer or kid's phone, with the emphasis on xbox. I like games, too, but don't use an xbox. Why do I need to see "xbox" on key apps, like music or videos? This isn't really a deficiency in functionality, but a marketing oddity. By all means have an xbox app, but don't conflate the xbox brand with listening to music on the phone.
7. WP's APIs are too locked-down. I've read probably 100 app pages on the WIndows Phone Store, and so many user comments ask developers for features the developers can't deliver because the APIs aren't available.
8. NFC capabilities show promise, but are nearly useless because of the lack of APIs. If you want to use an NFC tag to put your phone to silent, for example, you as the user still have to press a confirmation button, which takes away the whole point of automating through NFC.
9. No file manager. On Symbian and Android you can move files around to your heart's content. I realize that MS wants to protect the user from deleting system files, etc. Still, MS could let us manipulate our own data files, while still walling off access to system files.
10. Inability to attach arbitrary files to an email. This one really mystifies me.
11. No ability to use .m3u playlists. Again, these work just fine on Symbian and Android.
12. Most apps are updated far less frequently on WP than on Android. As said by others, the WP apps typically have fewer features than on Android.
13. The Windows Phone store allows for user comments, but not for developers to respond. I really like how Google has allowed developers to respond to every user review on the Play store. This allows developers to improve users' experiences, and also gives the prospective app buyer a sense of how responsible a developer is.
There's a lot to like about WP. The aesthetics are truly lovely. The phone (even the 521) is relatively fast. The live tiles are pretty, and seem useful (although I get identical functionality from home-screen widgets on Symbian).
I guess my overall feeling is that Google cares more about the user experience. They're constantly tinkering with their product to improve it. Microsoft has created a very pretty OS, but I feel like they'll constantly be stepping in my way to prevent me from doing what I want to do. I also feel like MS is not responsive to user requests, or at least not quick at it. It does sound like 8.1 will address a lot of users' biggest concerns, so that's a step in the right direction. I don't like the notion of Google using my usage data, and for that reason I'd love to go with a WP. But the inconveniences and bugs listed above seem too much to deal with.
I'll stay on my 808 until WP 8.1 comes out. Maybe enough APIs will be exposed to allow developers (or me) to create automated sound profiles. Maybe MS will fix the music file duplication bug, although they acknowledged it long ago and haven't fixed it over the past several OS updates. Maybe another manufacturer (Sony?) will finally come out with a cameraphone to rival Nokia.