Apple showed that a great interface can change the way you use a mobile phone, but Android's team didn't take this seriously until ICS. Android has never been about multitouch interfaces, intuitive apps, polished and refined user experience. Google spent a lot of time adding new features, powerful APIs, improving Android's performance, but the interface wasn't a priority. OEMs created their own interfaces, but something was missing.
Ice Cream Sandwich changed everything because it finally added the "soul" to the machine. The interface is consistent, there's a coherent design language that connects so many disjointed pieces and makes using an Android phone a pleasant experience. ICS is great because it removed Android's rough edges, it brought hardware acceleration, fluid and responsive interfaces that make you use the device more often.
Matias Duarte and his team certainly deserve the Gold prize for best system experience received at the 2012 User Experience Awards. ICS is not just "a major user experience step in Android's evolution", it's actually Android redefined.