I've been been looking more into Illustrator CS4 and have found a ton of new features that are really useful. The ability to use the scroll wheel when working with the layers pallet (very useful); the disclosure eyeballs over in the appearance pallet which allows one to see an object with or without as particular applied attribute without having to trash the attribute and then undoing the deletion of the attribute. These are just two more of the new features that I have discovered and like.
Also while learning CS4, when it comes to keyboard shortcuts I find myself going back and trying them within Illustrator CS3 because where I work has CS3 and haven't upgraded yet and probably won't for many years. I am finding that a lot of what I am learning within the tutorials for CS4 still hold true for CS3.
The blob brush is an interesting tool if one is creating stylized artwork, but on the production side it just seems a bit frivolous but I would have to have it and actually working on something that was for work in order to see any usefulness.
The addition of hyperlinks in various places that open up pallets (panels) for adjustment rather than having to go search for the pallet is also helpful. Especially within the appearance pallet.
Again, I have to say, that AI CS4 is probably the most awesome that they have come up with to date.