It's the seamless interoperability and synergy you get with your Apple devices. When you add a new product into the mix, they fit and work well together.

Your products also work well interacting with other Apple users and even 3rd party accessories.

They seem to manage the experience very well and plan out the 'what could go wrong'. But that leaves you having to do things their way. There are a lot of restrictions if you're more of a power user.

Sorry for the buzzword/jargon soup.