Let's take ten of the most popular new consumer technology products in recent years (with a couple of our portfolio companies in the mix): iPhone, Facebook, Wii, Hulu, FlipCam, Rock Band, Mafia Wars, Blogger, Pandora, and Twitter and let's try to describe in one sentence or less why they broke out (feel free to debate the reasons they broke out in the comments):
iPhone - mobile browser with a killer touch screen interface
Facebook - a social net with real utility
Wii - gesture based user interface for gaming
Hulu - your favorite TV shows in a fantastic web UI
FlipCam - a video cam that fits in your pocket comfortably
Rock Band - everyone can be a rock star for a few minutes
Mafia Wars - a natively social game built for social nets
Blogger - a printing press for everyone
Pandora - drop dead simple personalized radio
Twitter - blogging everyone can do in less than a minute
In most of these cases, the breakthrough product or service delivered a new experience to consumers that they had never had before.
From Fred Wilson's VC blog. The responses are great too.