Tim Bray The Mac Needs One More Key:
As I’ve written before, Windows is immensely better than OS X at keyboard navigation. This makes a real difference in the quality of the experience and especially the speed with which you can get work done. I’ve been thinking about how to work around this, and the only answer is another key on the keyboard, named Menu.
The Mac’s menu system, fixed in place and shared between apps, is in all other respects better than Windows - all they need to do is fix this one major botch and OS X would be miles ahead.
This can be done already, but it takes two keys, and is a bit clunky. Switch on Univeral Access Shortcuts, then Control & F2 activates the Menu, you make selections using the arrow keys (Escape cancels). There’s more in the AppleCare Knowledge Base article, Shortcuts for Activating Full Keyboard Access.