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I included fb2k mostly because it looks out of place by default on my Windows 7 machine. It did look a bit better on XP when I used it there before I went over to live in the Linux camp for over 7 years (where Amarok was my mostly used application).
I think that functionality is a very important aspect as well to make an application feel really integrated with the host operating system. For example, Stoffi will automatically scan the Windows 7 Libraries of type "Music" to find music files. It also uses Jumplists and Taskbar buttons. These are part of the UX design IMO and help provide that integrated feeling.
fb2k is not nearly as bad as Winamp or iTunes when it comes to disrespecting the UI conventions.
I haven't used fb2k for several years but I remember that I liked the way it was so "tiny" but had so much. My project looks way bigger but I am still inspired by its simplicity and its very small size (both as a download package and when installed).