Let's start off with the big change: Multi-tasking. The Android already multitasks, and does a better job than does iOS 4. I regularly listen to Pandora while doing many other things with my Droid, and have been ever since I bought the device.
It's nice to see that iPhone users can now do this...some of them, that is. iOS 4 won't allow the iPhone 3G to multi-task, so owners of that device are out of luck. And Apple has limited multi-tasking to seven specific kinds of services. Android does not have that limit. Apple says it imposed the limit in order to preserve battery life. I say give the user freedom to do what he wants, including running down his battery too fast.
iOS4 now lets you change your wallpaper. Once again, Android has had that for quite some time. So there's nothing to brag about there.
iOS 4 now has a task switcher, and yes, once again, that's been on Android for quite some time --- just press and hold the Home key. It's true that the Android's task switcher doesn't do quite the same job as iOS4's, because it won't let you end running apps. However, that's easy enough to do on the Android as well. From setttings, choose Applications --> Running Services, and you'll be able to kill services and apps you don't want running anymore.
iOS 4 lets you create folders as a way to group applications so that you can more easily find them. Here's one more instance where Android already does the same thing. And Android also has hot keys that let you launch any application you want without having to go through the bother of sorting through folders.
The iOS 4's Unified Inbox is also already available on Android. As for the iOS 4's new Exchange support, the Android supports built-in support for the Microsoft Exchange ActiveSync.
No doubt, iPhone users will be pleased to find all of these nifty new features. To Android users, they're old hat. For once when it comes to innovation, Apple seems to be playing catch-up, rather than leading the pack.