Android has always been criticized for its laggy OS all due to the UI running on the CPU of the phone, but ICS brings to the UI hardware acceleration to accelerate the overall. Nothing should actually be running on the CPU. It should all go to the GPU, because it’s an order of magnitude or two more efficient at handling graphic elements.
So now that Android 4.0 has hardware acceleration, it should easily run the whole UI at a silky smooth 60 FPS (obviously capped), and it should be able to save a bit of more battery life, too.
One other feature of Android 4.0 related to this is that all apps being built with Android 4.0 in mind (API14) will have hardware acceleration enabled by default.
In Honeycomb they didn’t want to break compatibility without the developer’s permission, so they left it to the developer to enable it in their apps. It’s only one line of code, though, so I’m sure it wasn’t too much of a problem, unless they really didn’t now they could do that.