The biggest addition is the inclusion of Game Center, Apple’s version of Open Feint. Game Center will include leaderboards, matchmaking, achievements, and more. It is still unknown whether older devices will be able to run the service, since Game Center support for the iPhone 3g and iPod Touch 2g was removed in iOS 4.1 Beta 3.
Apple may have also fixed the proximity sensor issue that was plaguing many iPhone 4s, which caused the touch screen to activate during a call. There is no official documentation of this fix in the changelogs, but many beta testers are reporting marked improvements.
Minor improvements include the ability to disable keyboard auto-correct, initiate FaceTime calls from an email address, and repositioned camera controls. Apple has also improved the API, allowing developers to access contact pictures to use in apps, along with a few other small changes.