Cortana isn't as helpful on my computer as it is on my phone but even then I don't really care about talking to my devices. The most helpful it has been is reminders, being able to say "remind me to call John at 5pm" is kinda nice.
Multiple desktops are very goofy on Windows 10. Not as good as on Linux or even OS X.
Microsoft's built in apps for media are pathetic still.
There are many annoying things also such as uninstall store apps constantly reinstall such as 3D Builder, Get Skype and Get Office. Even after I have Office installed!! Selecting a new Note from the Action Centre can only load the gimped built in OneNote not the full OneNote that is part of Office. Same with clicking the Surface Pen.
The start menu is seriously limited in where I can put things. I can't really personalise it much beyond putting tiles in the designated areas.
Edge is pretty awful with its current feature set.
Basically Windows 10 was released 6 months too early. I don't really understand why they were so desperate to release in July? Surely it would have made more sense to push back to October (Threshold 2 release window) or maybe even into 2016 when they can have a proper v1 version of Edge and more features in things like Groove Music which lacks even basic library management features like editing a songs tags.
Then we have the UI which is an inconsistent mess. Different styles all over the place.
So reading all this you would think I hate Windows 10 but I don't. I am disappointed with it as is but I am still using it over Windows 8.1 although it is more because it isn't worse than 8.1. At least not enough so to fall back to it anyway. It doesn't really give me anything amazing over 8.1 though.
I should add that I am not a Windows Store app user. I fucking hate how bland and limited most store apps are. There is no simple way to store app settings in a transportable way such as an ini file on a pen drive, for that you need to stick with old Win32 portable apps. For a tech professional/power user store apps are horrible.