I just bought my Lumia 1520 to replace my 920 last month. I love it, I love the phone.. Windows Phone 8.1 is nice and all but I am missing a lot of the core features of WP8 that made me fall in love with it..
First, like you mentioned, the hubs. The notification center is a really nice addition but the hub is basically where I used to go for notifications and I love how integrated Facebook was with WP back in the 8.0 days.
Another thing I really wish Microsoft didn't remove was the Facebook integration on Messages. I absolutely LOVE how Facebook messages and text messages were integrated into one app. When I first bought the 920, I did not know about this feature and when I saw how it worked I absolutely LOVED it. Another thing is Facebook is blocked in my office and the app basically bypasses it. I don't need the BS that is my news feed, I just needed chat and it worked flawlessly.
The very sole reason I switched from iOS to Windows Phone a year ago was because of the intuitive and good looking music player. Music+Videos was literally FLAWLESS in my book back then and I liked it more than I liked the music player on my iPhone but MS again changed it for the worst and this is probably the biggest annoyance I have in WP 8.1. Tags are horribly wrong, album art wont show up in ALL of my music (like WHAT).. Seriously I buy CDs and rip them on my PC (yep I still buy music physically, I'm a music snob lol just kidding, but I buy CDs not download them, I just like it that way).. I don't buy music on iTunes so I manually tag my songs and put album covers which is disappointing since my Windows Phone wont recognize my tags. Frustrating as it gets, I downloaded the Music Hub Tile but I absolutely wouldn't PAY for the app that is actually inside my phone, ridiculous.
To cut the long story short, I have mixed feelings with 8.1.. I love the notification center, Cortana, the tile backgrounds.. but in my opinion losing the core features where Windows Phone was supposed to be unique and stand out is a killer for me. But all of us have to move on from this.
I want to revert back to WP8 until MS gets it right with WP 8.1
As for me staying on Windows Phone, it depends. All I want is a music player that works, if that doesn't change then I'm really sorry, it's been great but I wouldn't really settle for anything less. Although hopefully Microsoft improves their music app.. I know they are doing their best but PLEASE.. It's been a frustrating couple of weeks with Cyan just because of the music player. Maybe you guys can't see the point of my rant but as a person who lives with music, it's really frustrating. I mean the app randomly stops playing music and the only way to get it to work again is to restart the phone. At the gym once, I had to restart my phone THRICE and that's on top of the wrong tags and slow player.
I will stay with Windows Phone if they give me a stable music player. I'm giving them one year (a long time just because I love WP) and if they don't give me what I need, I'm switching to Android, probably an HTC One (M9) if that's what it's called next year.. No iOS for me anymore, I am in no hell paying an extra 200 bucks for the 64 GB version.
EDIT: Oops. I thought this was about WP in general
Anyways, just read the first half and disregard the last two paragraphs
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