I am wondering whether you would consider spending a bit more time on Interface Builder. What I am thinking of is iTunes like interfaces - or interfaces similar to the winners of the Apple Design Awards this year.
Perhaps this deserves a book by itself - but there seems to be so many amazing interfaces that can be created for the Mac but very few hints around as to how.
Alternatively If there is a resource out there which already details this sort of thing I would be very happy to hear about it.
Thanks for your time, John