Just take a look at all the threads with 4.0.4 in the title. There are a ton of them within the first few pages. You'll have all the info, good and bad, that you need.
Personally, a few small things I've noticed are:
-Fade in from lock screen.
-Enhanced GMail actions, such as a dedicated Forward and Send key.
-Better battery
-Better signal
-Smoother multitasking/ Instant app switching
-faster browsing
-a bigger GTalk icon
-option to not lock screen until timeout after pressing lock
The biggest change is the "DOCK" in the system settings. Why you say this is so important?
Because when I select dock it reminds me that I don't have a dock. Why don't I have a dock? Because Google and Samsung think its funny to put charging contacts and dock software features on a 6 month old phone that doesn't have a friggin dock, car or desk, yet. And no a website picture of the platinum infused (expensive) desk stand doesn't count.