I guess I'm equally baffled by hearing from people that love it. :) But just to give you some of the more frustrating examples:
1. Sometimes swiping from a particular corner or edge has one function...sometimes it does something completely different. And this changed from screen to screen. And there was no rhyme or reason I could deduce.
2. Switching between apps was a nightmare...sometimes I could pull up what I assume was meant to be a multitasking tray on the left, but often I couldn't. And I'm in good company when I say that I never understood how I was meant to close an app. I would think an app was closed, only to see it show up in that elusive multitasking tray even after closing it.
3. The settings were laughably spartan and almost useless in some cases. For the first time ever, windows OS gave me much less options than my Mac OS.
Truthfully, I can't now remember the most egregious problems in much detail (mental block?? :) because after about three months I finally threw in the towel and reinstalled win7, which I love. Don't get me wrong, I love innovation, and I've been jacked into a computer virtually every day since I was 16. My daily stuff requires me use everything from photoshop/illustrator to AfterEffects to 3D software to accounting software etc. Definition of a power user/ geek. :) But it feels like Microsoft started from scratch with win8...in the process, they threw out even the GOOD lessons learned over the last 30 years.