It's a single block of metal laser cut to fit all the components within a solid yet light fixture. It's a power plug that if tugged will not drop your laptop to the ground. It's a backlit keyboard that feels great on the fingers. It's a beautiful screen that you can't get anywhere else. It's a large sized trackpad that answers to your fingers without noticeable lag. It's a keyboard that has all the keys on the top row to do all the things we do on a regular basis. In short it's a super well integrated machine that wakes up from sleep in an instant, has great battery life and is well suited for a worry free computing experience.
It's one of the greatest industrial designs, with thought of tons of details, very solid (and expensive to perform) unibody construction from a block of aluminum, and several Apple-only or Apple-makes-better things thrown in, from the hi-dpi display (that's better than anything hi-dpi on comparable price laptops) to the mag-safe adaptors, to thunderbolt, to the multi-touch glass trackpad, to the magnetic lid etc.
The other replies explain the benefits of Apple hardware but I have to emphasise the trackpads - every other trackpad feels like dragging my fingers through mud in comparison and the gesture-handling is really slick.