While musing the responses here, an observation came to me that in many of the single-word/phrase situations we list, it is often not the word itself that has power, but something else. In Disney's Aladdin, the LOTR, and your run-of-the-mill magician's hat, the terms "Open Sesame" (I forget if that's right and too lazy to look it up), Mellon, and Abracadabra are not so much powerful as they are recognized by something that is: a magic hat, a secret hideout's guardian, or a magical dwarven door.
Perhaps this is off topic, but in many cases of magical stuff, there is a coincidence factor. The best example I can think of is in the Hobbit, where one must have the key when the thrush knocks just as the last light of the setting sun shines upon the keyhole. That's a lot of parameters - especially when you can only read these instructions by reading the map under starlight.
