So, a number of people have commented about the inability to navigate GUI's completely via the keyboard (without having to use the mouse). I don't find using the mouse that much of a burden, but there are times (especially when filling out forms in websites) that I'd like to do things like pick my state from the choice/combobox without having to go to the mouse.
It turns out you can turn on "full keyboard access" on the Mac and do this. It's available in Mac OS X 10.3 and for all I know it was there in earlier versions as well. If you open the System Preferences and choose "Keyboard & Mouse", there are three sub-panels labeled "Keyboard", "Trackpad", and "Keyboard Shortcuts". If you navigate to "Keyboard Shortcuts", there is a checkbox at the bottom that says "Turn on full keyboard access". If you check this, you can navigate menus, lists, etc. using the keyboard.
Brad Shuler told me that Weiqi Gao blogged about wanting this but I couldn't find the relevant blog entry. Brad also told me there are more fine-grained control options available under Tiger (10.4). Maybe he'll describe them in his blog.