Great products disappear
Recently, I lost my Nexus S and replaced it with a Venue Pro. Microsoft got a lot of things right with Windows Phone 7, compared to both iOS and Android. (I used an iPhone for two years before the Nexus S.)
One particular detail struct me as insanely clever, but I didn't even notice it until I described to my girlfriend what was going on. When I pull my phone out of my pocket after getting a notification, the screen turns on. Make no mistake, this is a tiny detail. But it makes a huge difference. It's possible to see what the notification is and unlock the phone without hitting any hardware buttons.
To me, that's emblematic of Microsoft's attention to detail with Windows Phone 7. The phone knows when it comes out of your pocket (thanks to its ambient light sensor). What else are you going to do with it when you pull it out of your pocket? Why should you need to hit a button to bring the screen up?
Noticing this detail about my phone made me aware of a bigger concept: well-designed things are imperceptible. They do what they're designed to do so well that you don't even think about it.