Most of the changes are under the hood, and unfortunately Samsung is covering the now rather pretty ICS with a almost unmodified version of Touchwiz. GPU acceleration for the OS was one of the biggest talking points when ICS was launched, and if you look in the settings, you will that it indeed is present. The GS2 is so fast a phone though that it really doesn't seem to make a difference, you can't make super smooth even smoother. On other phones, mainly HTC ones that had Sense 3.0 and Sense 3.5, you can clearly see the difference, those phones run much smoother after the ICS update. The task manager is different, you can swipe apps off the screen to close them. The browser is new and improved and I believe they finally added a rotational transition when going from portrait in landscape mode, which is mostly noticeable in the browser since Touchwiz still doesn't support landscape mode. You get face unlock as a security feature, seems more like a gimmick if you ask me, would love to see someone unlock their phone in the dark. Another nice added feature is to be able to log and see your data usage. ICS has a nice way of making folders, iPhone style, just drag one app over another to create a folder, but from what I have seen Samsung took that out since it has it's own build in folder feature already built into Touchwiz.
On every leak you were able to remove the default setting from Touchwiz from within "manage applications", but apparently this ability doesn't exist in the final official build, keeping us from using the default ICS launcher instead of Touchwiz.
I am sure there are a lot of other features in ICS that I didn't mention, but that's all I can think of from the top of my head.
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