Am wondering if anyone has any recommendations on what GUI library can be used for developing cross platform applications such as Spotify. I have a feeling Qt-Quick might be the best answer but am not aware of most of the newer UI technologies so if you have any recommendations please feel free to share.
Well - you already mentioned Qt in the question - but that's my answer.
I would very strongly recommend Qt as a cross platform development UI toolkit. This comes from personal experience over 10+ years in numerous types of applications in different industries and platforms.
The toolkit in no way feels aged - and (I don't think you were actually implying this, but...) I don't think there's any question of not being able to produce modern slick looking applications.
Intuitive, ease to use and flexible. Excellent documentation, well discussed, with many examples and forums. Fully supported of multiple OS desktop, embedded and mobile platforms. These are just some of the reasons it's a great option.
-- edit -- This article about mflow has plenty of relevance. I quote:
Here's a screenshot of the mflow downloadable desktop app:
I would have to suggest using Silverlight; as far as Windows and Mac users, it will Just Work™ for everybody, and I'd hazard a guess that most of the serious Linux users out there are well-enough versed to install and configure an alternate runtime like Moonlight on their systems.
Also, with Silverlight 4, you can create out-of-browser applications that appear like seamlessly installed desktop applications, as opposed to web plugins.