Roger - This was great. Love your work but this one was especially pragmatic. And appreciate the examples from both B2C and B2B.
It strikes me that there's two pieces to a good promise - articulation and evidence.
Half the battle is simply figuring out what to say. Many companies get stuck because they can't articulate their promise clearly. But even among those that do, I've noticed many don't include compelling reasons to BELIEVE the promise. If I'm a potential consumer, I'm thinking: "Ok... can you prove it? Can you give me evidence that this will be worth it? Can you SHOW me how it was worth it for others?"
A good promise is as much signal as it is message. The message is what you say. The signal is how you back it up. Especially in my world of B2B, I notice a lot of businesses going 0 for 2.