In Mac OS, I really wish I could toggle between dialog buttons (e.g. Save / Cancel) using the arrow keys on the keyboard instead using of my mouse (like Windows - hate to say it). Maybe it's already possible to do this by changing a preference somewhere, but no one I've asked knows how, and I couldn't find anything in the missing manual.
Similarly, when "tabbing" through a form in a web browser, I wish it wouldn't bypass dropdown menus. I want to be able to access dropdown menus with my tab key and control my selection with the arrow (like in Windows - again, hate to say it).