Best feature for me on iOS 5 is iCloud and all the services it's providing. iMessage, contacts saving, free email account, calendar, photos and videos sharing via the cloud, reminders, documents which allow me now to throw away the USB flash card (I can access my docs anywhere anytime, and ofcourse while doing docs both on iPhone MacBooks with iWork it auto save all your work so no need to save every so often like on windows. The 5gb of free space, the ability to backup most of the files and even the phone settings on the iCloud, the music matching thing.
The iCloud is really what I'm enjoying on the iOS 5 and I'm taking full advantages on it.