Android 2.3 Gingerbread vs. Android 4.0 Ice Cream Sandwich
A few additiona not mentioned in the above article that immediately come to mind: the ability to take screenshots, the option to freeze any app from running (including bloatware), ICS-only apps (such as the vastly improved Gmail), and Webtop 3.0.
And most of the under the hood changes are not just for developers. The changes represent noticeable technical differences that have an impact on how your phone actually performs:
Learn about the technical differences between Gingerbread and ICS
There's more to ICS than just the above, but that should be enough to show that it's far more than an incremental update.