They are indeed meant to be discovered -- they are usually listed on menu items. You can hold down modifier keys like Option, Command, Ctrl and Shift to see how the menu item's text actually changes to reflect the action that will be taken if you press the respective key command. Very discoverable. I've never seen any other OS that does this (not that I look for it specifically). Also note when ellipses at the end of a menu item disappears when you are holding Option or whatever -- this means the action will be taken immediately, instead of asking the user to confirm (quick example is on "Empty Trash" in the Finder menu).