Disclaimer - I'm not in the "OMG HORRIBLE" camp. It's a usable interface. It's just not
quite right.
Why are people complaining about It? Is it because of how it looks? (I quite like it with its minimalism)
Honestly, I'm more interested in function than look. I'd prefer it to be messier, if that made it more functional. Minimalism can be a good thing (complexity, in and of itself, isn't necessary a virtue), but just doing it for "ooooh, pretty!".... meh.
It's the generally low navigability. I didn't have much of an issue with it until I took up enchanting, but when I did, man did it come down hard what a PITA it is to navigate through a menu designed for a console on a PC.
This is the big part. Many parts of the interface are pretty clearly designed around the controller's "push thumbstick to slide" action, rather than the mouse's "click and drag" action. While the mouse and keyboard can approximate the thumbstick actions, it's somewhat awkward.
There are also some parts that just aren't designed well, regardless of what you're using.
Container interface, for instance - (keyboard commands) When you're putting items IN a box, "R" puts
one item, "E"/mouse click
uses an item. When you're taking items OUT of a box, "R" takes
ALL the items, while "E"/mouse click takes one.
This is inconsistant design - it can lead to a variety of annoying mistakes. Like drinking those potions / eating those alchemy ingredients, when you click on them (you were just clicking to TAKE stuff, but can't click to PUT stuff). And because of that training you to avoid clicking on things when in a box, you get into the habit of pressing R constantly to PUT stuff.... and then keep pressing it when you go to TAKE, and you get all 800 pounds of alchemy ingredients that were in your storage box. Which you now have to put back in, using many presses of R.

So yeah - it's a usable interface. But there's clearly room for improvement.