[wslh]

I think developing a good UI is hard. More when you develop a multiplatform application like Calibre.

Probably QT is currently the best way to go but the aesthetic is not solved by the framework alone.

[dredmorbius]

I could care less what framework is used, and am fairly positively inclined toward Qt in general.

Some frameworks (GNOME in particular) lead to a shit-ton of user-hostile design decisions.

How you use the framework matters a lot. This is where Calibre needs a lot of love. The interface isn't intuitive (I've got v1.25 open now, and ... it's a mess).