Find My Android
From my point of view, the best thing about iOS 4.2 for the iPhone is the addition of Find My iPhone, a free service that finds your iPhone if you lose it, and remotely lock it, wipe it, or display a message or sound on it.
Find My iPhone was previously available to those who paid the $99 annual fee for a MobileMe subscription. Here, for the first time, it's available to everyone for free.
Google would do well introduce a similar feature and service for Android. It certainly would be easy to do. And given that the Find My iPhone service uses Google Maps to show where a missing iPhone is, adding the service to Android would mean more people would use Google Maps.
By the way, you may have noticed a Droid commercial that touts what appears to be a similar app for Android, called Lookout Mobile Security. That app, though, had to be downloaded, isn't built into Android, and if you want to use features such as lock and wipe, you'll have to pay for them. So it's not nearly as good as the free Find My iPhone.
Android phones are essentially full-blown computers, but they're missing a very basic computer feature --- the ability to print. iOS 4.2 lets iPhones print Web pages and more, although that feature is severely limited, because it works with only a handful of specific wirelessly enabled printers.
Google should introduce a similar feature but do Apple one better, and include a feature Apple originally planned to include in iOS 3.2, but then dropped --- the ability to send a print job to a PC or Mac, and have them do the printing. That way, you would be able to print to any printer to which your Mac or PC has access.
For more details about the iOS 4.2 update for the iPhone, check out my review, iOS 4.2 on the iPhone 4: Nice but not revolutionary.