For me, it's a couple of things really, some I will get past, most are petty. I like the new task switching, it's very swish. Vertical Start Menu, it's fine.
I don't like the action centre much. It is good that you can turn off a lot of the notifications, as some you just don't care about.
On the Charms bar I'd use it to swipe in and see what the time was, usually when I'm watching Netflix late at night, to make sure that it's not too late. That option is gone, unless I have a task bar showing all the time.
Also, on the charms bar there were Wi-Fi and brightness options that were usable. In Windows 10, it's more like a phone. When I tap the Wi-Fi button, it turns it on and off. I have 3 wireless networks in my home, to select another I have to do a long press and another option pops up, pick that, and it will open control panel. In Windows 8, you'd tap and it would open the wi-fi options.
With brightness, you tap and it moves to 25, 50, 75 and 100%. I want 60% as that's 120 cd/m2. To do this I have to tap and hold, and go to another screen. In Windows 8.1, you'd hold and drag.
If you're in Desktop mode, when you're in a modern app, there is a back button at the top left of the screen. I use both a Surface and a Desktop machine. I doubt very much that I'm going to use the lower back button, simply because I use my Surface less.
Windows 10 for tablets has got a lot better, it's just at the moment, I still prefer 8.1. I think time will change that.