Although it my seem to be the most obvious design principle, the notion of "Start with the Customer Experience" can be, and often is, the thing that businesses and product development teams can forget about during the development phase.
In other words, just because you started at the Customer Experience, that doesn't necessarily mean you'll get the right end result. You need to stay there.
I've seen it happen many a time: teams start at the customer UX design - it's awesome, it's simple and it "just works" beautifully. Then it comes time to develop the technology and stuff behind the scenes to make it all happen. And during that process, there are hundreds, maybe thousands of design decisions being made that create opportunities to lose sight of the original experience design...questions around which standards to use, which stack to employ, the resources and skills available, competing priorities and resource allocation, complexity vs. simplicity of implementation, feature phasing and sequencing, time constraints, pressure to get to market, etc, can often become the most important things that teams spend energy in solving, rather than remaining true to the original design. And before you know it, you can look at the end result, compare it to the original vision and say: whoah! - that's a long way from the original vision! We've all been there.
The companies, product companies and experience providers that stand out and consistently deliver awesome customer experiences are the ones that don't compromise on the original vision and design. They start with the customer experience and they stay there. Apple, of course, is the leading example of this discipline. Reminded by Brad Feld, here's great clip from Steve Jobs a few years ago at the Apple WWDC in 1997...
Here he talks about starting with the customer experience and about how then they figure out the technology to make it happen.
But what he doesn't say but is plainly obvious, is not only how they start there, but how they stay there with the customer UX throughout, ending the development lifecycle with a product that is as awesome and true to their original vision as when they first set out on the design journey. That's the magic of delivering awesomeness, not just designing it.