Brand promise is the collective experience people have with your brand once you combine all other factors of your brand foundation. So it's not tactical or directive. It's the essence of the brand.
Brand Building Fundamentals: Positioning
Brand Building Fundamentals: Desired Consumer Experience
Brand Building Fundamentals: Points of Difference
Brand Building Fundamentals: Points of Parity
Brand Building Fundamentals: Brand Personality
Brands can be fun, free willed, playful, studious, a work horse, etc. But those attributes don't drive much energy into the brand. Worse, your interpretation of "fun" may be decidedly different than mine. So we could work on the same brand using "fun" as the brand personality and end up in vastly different areas.