ICS/4.0 has more GPU acceleration, but I'm not entirely sure where exactly it does and doesn't apply. I know on the ICS ROM I'm running it's definitely applied to the native browsing engine, so the stock browser (and others that build off of it, like Dolphin) are actually very smooth now. Just a hair short of iOS-like. I would imagine the regular assortment of gApps are similarly enhanced - the new gallery in ICS is great - but again, I'm basing this off of my experience with a community ROM for older hardware (Galaxy S Fascinate). I messed with a Galaxy Nexus at my local VZW store and it actually seemed just a tad less smooth than my own phone - perhaps because it's pushing more pixels? Who knows.
In a nutshell, that uncertainty over what kind of experience you're going to get is the biggest problem with Android, IMO. With iPhones and iPads, you know exactly what you're getting.