I dont think the UI is ugly. The MS ribbons UI, THAT's ugly!
What I appreciate about the UI is its accessibility. Everything is big, or can be made big. The UI scales up to higher DPI correctly, without things getting cut off.
On windows it looks pretty much like a native app, albeit one without modern fancy effects like mouse overs, but I hardly care. And arent we moving away from all that anyway? I think trying to make it look more native on other OS is worthwhile, as it looks a bit out of place otherwise. But again, it's a minor issue, IMHO.
Skinning, round corners, glowing things..., where they are not natively supported, I associate these with sluggish performance, memory hogging, fixed UI elements that do not scale and other accessibility issues. Just say no(TM)
It's not that I don't care how things look. I spend a lot of time tweaking books to fix formatting problems most people wouldn't care about. And I appreciate an attractive, well designed UI like in many iPad apps I have (though none of them approach the complexity of calibre). How calibre looks is not high on my list of concern. I did replace a couple of icons and am now quite happy.
That said, there are a few things ... (aren't there always?
1. Is there a way to put the book detail panel on the left side?
2. Set the face and size of the font in the book list, independently from the font for the rest of the UI?
3. Import a recipe by drag n drop into the main UI, or double clicking.
4. The add a custom recipe dialog always asks if you're sure you want to close, there may be unsaved changes, regardless whether there are any or not. ( It's also not immediately obvious how to save changes). Either it should ask only when you really have unsaved changes, or just assume you know what you're doing and ask nothing. This is where "don't ask me again" comes in handy.