Definitely a better multitasking UI (I said UI, not the API or how it works). Double-clicking the home button to bring up a row of icons -- which you can only see 4 at a time -- isn't cutting it.
And a better way for apps to communicate with each other. 'Work on a file in one app and open it in another' kind of stuff.
I love the multitasking UI in iOS. Just swipe with four fingers to get back to the previous app. No taps, clicks, or nothing else. Just a single swipe. Much faster and more intuitive than any other OS for that.