Well iOS 4 has been out for a while now and in a few words it is fantastic, But in a few more words here you go:
Multitasking: It works great the way that Apple has done it. Yes it is not true multitasking but it is great because it does the important parts in the background such as audio playing, task completion, GPS, VOIP. Also if the app is updated for iOS4 then it well remember your spot in the app. Which means that to make the most out of an app it has to be rewritten or updated for iOS4.
Folder: Folders are great they allow you to organize your apps and screens. All you have to do is drag apps on top of each other and that creates a folder that gets auto named based on the types of apps that they are. But you can also rename any folder to what you choose. Also you can only but in 12 apps in the folder.
Background wallpapers: Not a lot to say here other than you can now set background wallpapers.
Those are basically my top fav things in the update.