WiFi sync, iMessage (I can "text" my parents for free!) and notifications.
I don't include all the iCloud stuff because that's not really iOS 5. It's a whole other level of awesome that adds on top of WiFi sync. When you have an iPhone and iPad and only four USB ports on your iMac, you really tire of the sync game. Now I could possibly buy a charging dock and get some cords off my desk. I'm all for getting rid of cords, which is why if you have the money to give me for an entirely wireless home theater system, I will be your best friend. HDMI is OK, but get rid of my speaker wires!