[Filburt]

Note: An edited version has been sent to Apple as well.
Although Aqua is easily the most advanced commercial windowing system available, Apple has neglected to address one very important issue: not respecting the display's DPI (dots per inch).
Majority of Apple's own displays maintains consistent DPI of around 100, from 96 DPI for 17" Studio Display to 106 DPI for 12.1" iBook (only 15" iMac and 14" iBook deviate significantly, at 85 and 91, respectively). And it seems Aqua has been designed for displays with 100 DPI. Although I think it's great that using an Apple display means relatively consistent user experience, elsewhere in the spectrum, higher resolution is gaining popularity (much like the gigahertz CPU war).
Don't get me wrong, my eyes can't handle 133 DPI that some of the PC notebooks offer these days (Dell's 15" UltraSharp LCD notebooks immediately come to my mind). But it's great to have extra screen real estates. That is, if the user interface (Aqua) can dynamically resize some of the screen elements (mainly fonts) to accommodate increased DPI. That is, a 72-point font should measure 1-inch regardless of the resolution. Plus, fonts should look extra smooth and detailed with added resolution (without the soft heavily anti-aliased look).
In the Windows world, increasing the DPI is an option, but it is poorly implemented. Changing the DPI on Windows XP, for instance, can make many dialog boxes and such look, well odd. Apple should do this, and do it right.
In the professional field, you can pick up ultra resolution displays, such as Viewsonic's $8000 22.2" display with 3840x2400 resolution (204 DPI). It is conceivable that in perhaps 5 years, such displays can be priced at a level similar to HD Cinema Display today.