The awful email experience. It's. The. Worst.
I like emails on WP.
'Report spam' button would be useful, but other than that it's pretty great. I get everything done just fine.
What bugs me the most is that hubs have been removed, accessing lenses take two taps in Nokia Camera, clipboard is really short, I've had copied text lost so many times already.
As mentioned before, background downloading would be nice. I really don't have anything useful to do while I'm waiting for 130MB of maps to download.
Better support for background services, like SIP. WP can act like a SIP phone only if you leave the app running in foreground, if you so much want to check emails, you're offline. It doesn't bother me for personal use, but I know if I get a chance to choose my work phone in future, I'm picking up an android (xperia, preferably)
It really bugs me that they change how things work in OS. Great, there are hubs, awesome, I need three tiles on start and have quick access to all media and games. Now I need 4891 tiles to achieve the same. Well now we got (quite great, with few bugs) folders, so at least there is that.
Skype app is TERRIBLE! I can't believe this is a M$ app running on M$ OS, and it's awful. They need to fix it ASAP.
Oh, did I mention I miss hubs?