A few minor UI gripes:
1. Hold Shift while clicking on a Finder window’s toolbar display toggle widget (the Tic-Tac shaped one in the upper right). Handy dandy. Now try this in any other application with a toolbar. It doesn’t work. Phooey.
These are surely not the biggest of UI problems, but they are inconsistencies that ought be corrected. Supposedly, the Cocoa and Carbon frameworks did at one point behave identically, but this was broken before the initial 10.0 release.