Left: Cupcake (1.5). Right: Ice Cream Sandwich (4.0)
Again, while it just plain looks a lot nicer now, things are much deeper than what can be shown in a screenshot. There's a lot more hardware acceleration across the board - transitions that before were stuttery or abrupt now have (semi-fancy) transition effects that are buttery-smooth. Navigating and using the phone on a daily basis is more intuitive and just plain more fun. Consistency across the board has been improved significantly (though there's certainly more to be done on that front), and customization is easier now than it ever was before.
For most of us, Cupcake (and even later versions of Android like Éclair, Froyo, and Gingerbread) probably weren't polished enough from a user experience perspective to be something our parents would find easy and enjoyable to use (granted, there are exceptions for those with more tech-oriented parents). But what about now? With ICS, is vanilla Android finally polished enough to hit the non-techy world? (Note, we're talking about vanilla Android here - don't factor in manufacturer customization such as HTC's Sense, Samsung's TouchWiz, or Motorola's Blur variants.)