What has changed in iOS 4.2?
There were quite a few changes. 

The biggest is that this is the first version of iOS 4 for the iPad, which brings multitasking, folders, unified inbox, air print, air play, and more. 

This is also the unifying iOS so that the iPad and iPhone and iPod Touch are running the same OS now.

You can find an entire list at http://www.apple.com/ios/