The UI for PC is a horrible mess:
- the developers kept the 'console' idea of navigating with a controller (which works via the keyboard), then added the idea of clicking with a mouse. Sadly sometimes the PC thinks your mouse-click didn't hit any of the options, and in that case it reverts to whatever it thinks the controller selected (why it thinks you didn't click on anything is another issue).
- a related issue is that left-click=skip dialog. However, due to the stupid issue above - if options appear during a conversation, and you click anywhere else on the screen, the game appears to assume that you meant to click on the default option

.
- if I press 'q' - the game pauses, and I get a list of 4 favourites, in a tiny font that I can scroll through. My screen is 1920x1200 and I get a modal dialog box the size of a postage stamp

.
- tab=close, most of the time. Quests are weirdly on the 'escape screen', where tab!=close.
- quest screen in general. From reading reviews, it seems that it's not very intuitive how 'miscellaneous' works - even for seasoned gamers.
Beyond that, it's just a case of a dreadful UI for a mouse along with generic bad design. Not that oblivion was a great example, but in 2011 this is embarrassing.
- the map deserves it's own mention, but I assume it's basically useless for everyone - paths are not visible, height only becomes apparent if you mess with the zoom options for a while.
- ditto that an option turns grey when you've asked a question. It does not mean that shouldn't ask the same question, again and get a different answer, eventually leading to a quest

.
- inventory management. If you stick with this design, a better layout is shop on left, player on right. Rather than one on top of the other (anyone else purchased things that they thought were selling?).
- inventory management[2]. Left click = transfer item (purchase/sell/pick-up), except when looking at your own inventory (where it's consume, R for drop). That's understandable, but when a chest is opened, clicking on your own inventory to transfer it to the chest instead consumes the item - this is really annoying. We don't need to equip/consume items when looking in a chest!!!
- "stealing". Not really UI, but when you become friendly with an NPC, and they let you take their health potions. Move your cursor 2 pixels when clicking to pick up the potion, and you tend to get a bounty put on your head after picking up a 1 value arrow on the same surface... we have lots of keys available - we don't need 'left-click=do everything'.
[I'm fussy on UIs, but this UI doesn't even try]