This isn't quite a mind-blowing feature, but it might come in handy to those who frequently need to track down a specific menu item or command. In the menus of any apps (including even menubar utilities), you can simply type the first letter of an item you're trying to find in the currently selected menu. This unfortunately doesn't seem to work across all menus, i.e. - you can't type a command to find it in any menu in the current app, but it should at least help with those "oh I *know* it's in this menu somewhere" moments. This also appears to work across all apps, Carbon (Finder, Firefox) included.
Oh a broader scale, this trick works in many apps in various ways. iTunes is perhaps the best example: you can type the first few letters of an item you're looking for in your media list (or in the Sources list on the left), and iTunes will take you to that particular section, based on which column you're currently sorting items by, or which Browse column you have selected.