Whitespace is necessary for ease of use, and comfortable reading. The Aqua interface is very good at blending into the background, for heavily content-based programs (like Mail, TextEdit). The Brushed Metal textured interface is purposefully distracting. No element is given precedence over others - data, interface widgets, titles - all arranged amid the interface. Safari is an exception, because, unless (like me) you have the Status Bar enabled (off by default), the only brushed metal is concentrated at the top, where the sheer number of elements demands something less spaced out and separated than the Aqua toolbars (see Jaguar Finder).
I love how the OS X interfaces are sorted out. There's a nice balance of white and grey, both of which go brilliantly with the blue widgets (like the scrollbars), and in the end, the whole interface fits together very nicely.
Compared to how Windows gives their developers more control over the way their windows look (easily), Apple's Interface Builder allows developers to, intuitively, develop windows that follow Apple's Human Interface Guidelines.
I can't complain at ALL with the way Apple produces its products, although I would like Mail to be updated a little bit, functionality-wise, I don't feel it needs ANY modification interface-wise (short of the Dock-context menu, which is woefully understocked).
If people don't like Aqua applications, I'm sure there's a program which does the opposite of DeMetalifizer.
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